Separation process and apparatus for light noble gas

ABSTRACT

Process and apparatus for producing helium, neon, or argon product gas using an adsorption separation unit having minimal dead end volumes. A second separation unit receives a stream enriched in helium, neon, or argon, and a stream is recycled from the second separation unit back to the adsorption separation unit in a controlled manner to maintain the concentration of the helium, neon, or argon in the feed to the separation unit within a targeted range.

BACKGROUND

The present disclosure relates to the recovery of a light noble gas froma gas mixture containing the light noble gas and at least one othercomponent. The light noble gas may be helium, neon, or argon.

A variety of processes and techniques have been developed to separateand recover light noble gases from multicomponent gas streams. Suchprocesses include stand-alone membrane separation units, stand-alonecryogenic units, and combinations of membrane separation units,cryogenic units, and pressure swing adsorption (PSA) units. As usedherein, the term “pressure swing adsorption” includes “vacuum swingadsorption” and “vacuum pressure swing adsorption.”

Disclosures related to such processes and/or techniques includeWO2016/096104; DE102007022963; and U.S. Pat. Nos. 3,250,080; 3,324,626;4,077,779; 4,690,695; 4,701,187; 4,717,407; 4,783,203; 5,542,966;8,152,898; 8,268,047; and US 2017/0312682.

It is desirable in the industry to recover light noble gases fromvarious feed streams that contain the desired light noble gas.

For example, it is desirable to recover helium from a feed stream (e.g.natural gas) having a low helium concentration, e.g. from 0.1 mole % to4 mole % or from 0.1 mole % to 2 mole % helium, or from 0.1 mole % to 1mole % helium. Other example feed streams include nitrogen rejectionunit (NRU) vent streams, CO₂ liquefaction vent streams, recycle streamsin manufacturing processes, recovery streams in airship fillingprocesses, reboiler non-condensable vents in air separation units, highpressure gaseous nitrogen (HPGAN) from air separation units, reflux tolow pressure columns in air separation units, or liquefied nitrogenstorage tank vents.

It is desirable in the industry to recover light noble gases from a feedstreams where the concentration of the light noble gas varies over time.

It is also desirable in the industry to produce a product gas containinga light noble gas within target concentration specifications for feedstreams where the concentration of the light noble gas varies.

BRIEF SUMMARY

The present invention relates to a process and apparatus for separatinga light noble gas from a feed gas stream comprising the light noble gasand at least one other component.

There are several aspects of the invention as outlined below. In thefollowing, the reference numbers and expressions set in parentheses arereferring to an example embodiment explained further below withreference to the figures. The reference numbers and expressions are,however, only illustrative and do not limit the aspect to any specificcomponent or feature of the example embodiment. Components and featuresof any embodiment may be combined with one or more components orfeatures from one or more other embodiments, and all such combinationsare considered to be within the scope of the present invention. Theaspects can be formulated as claims in which the reference numbers andexpressions set in parentheses are omitted or replaced by others asappropriate.

Aspect 1. An apparatus for producing a light noble gas product from afeed gas (11) comprising a light noble gas and at least one othergaseous component, the light noble gas selected from the groupconsisting of helium, neon, and argon, the apparatus comprising:

-   -   an adsorption separation unit (10), wherein the adsorption        separation unit (10) comprises        -   a plurality of vessels (100 a, 100 b, 100 c, 100 d, 100 e)            each containing a bed of adsorbent;        -   a feed gas header (200) in selective fluid communication            with each of the plurality of vessels (100 a, 100 b, 100 c,            100 d, 100 e);        -   a product gas header (210) in selective fluid communication            with each of the plurality of vessels (100 a, 100 b, 100 c,            100 d, 100 e);        -   a tail gas header (220) in selective fluid communication            with each of the plurality of vessels (100 a, 100 b, 100 c,            100 d, 100 e);        -   process gas transfer lines operatively connecting the            plurality of vessels (100 a, 100 b, 100 c, 100 d, 100 e) to            the feed gas header (200), the product gas header (210), and            the tail gas header (220);        -   each vessel (100) of the plurality of vessels (100 a, 100 b,            100 c, 100 d, 100 e) having process gas transfer lines            associated therewith (101, 102, 103, 104, 105, 106, 107,            108);    -   a plurality of valves in the process gas transfer lines        including a plurality of valves adjacent and associated (110,        111, 112, 113, 114, 115) with each respective vessel (100);    -   wherein the adsorption separation unit (10) has a central        volume, V_(c), of process gas transfer lines (101, 102, 103,        104, 105, 106, 107, 108) associated with each of the respective        vessels (100);    -   wherein the central volume for each respective vessel is the sum        of        -   (i) the volume contained in the process gas transfer lines            associated with the respective vessel connecting the            respective vessel to each valve adjacent (110, 111, 112,            113, 114, 115) the respective vessel (100),        -   (ii) all dead-end volumes (109), if any, connected at a            junction to the respective vessel (100), and        -   (iii) all dead-end volumes, if any, connected at a junction            to any of the process gas transfer lines associated with the            respective vessel (100) that connect the respective vessel            (100) to any valve adjacent (110, 111, 112, 113, 114, 115)            the respective vessel (100);    -   wherein the central volume for each respective vessel includes a        secondary volume, V₂, where the secondary volume is the sum of        -   (i) the volume of all dead-end volumes (109), if any,            connected to the respective vessel (100);        -   (ii) the volume of all dead-end volumes, if any, connected            at a junction to any of the process gas transfer lines            associated with the respective vessel (100) that connect the            respective vessel (100) to any valve adjacent (110, 111,            112, 113, 114, 115) the respective vessel (100), and        -   (iii) the volume of any process gas transfer lines (108), if            any, having a first end terminating in a valve adjacent            (115) the respective vessel (100) that is configured to            permit transfer of process gas to the tail gas header (220)            when open and having a second end terminating at a junction            to any other of the associated process gas transfer lines            (102) that connect the respective vessel (100) to any other            valve adjacent (110) the respective vessel (100); and    -   wherein the secondary volume V₂ is less than 5%, or less than        3%, or less than 1% of the central volume, V_(c), for each        vessel (100).

Aspect 2. The apparatus according to aspect 1 further comprising:

-   -   a second separation unit (20), the second separation unit (20)        having an inlet, a first outlet, and a second outlet, the inlet        in fluid communication with the product gas header (210) of the        adsorption separation unit (10);    -   a gas mixer (60) having a first inlet for receiving a stream of        the feed gas (11), a second inlet in fluid communication with a        source of a second gas (17) having a higher light noble gas        concentration than the feed gas (11), wherein the feed gas        header (200) of the adsorption separation unit (10) is in        downstream fluid communication with the outlet of the gas mixer        (60);    -   a sensor (50) in at least one of (i) a process gas transfer line        (11) supplying the first inlet of the gas mixer (60), (ii) a        process gas transfer line (12) connecting the outlet of the gas        mixer (60) to the feed gas header (200) of the adsorption        separation unit (10), and (iii) the feed gas header (200); and    -   a controller (80) in signal communication with the sensor (50),        the controller (80) operable to control the flow rate of light        noble gas from the source of the second gas (17) to the second        inlet of the gas mixer (60) responsive to signals from the        sensor (50).

Aspect 3. The apparatus according to aspect 2 wherein the secondseparation unit (20) is an adsorption-type separation unit, amembrane-type separation unit, or a distillation-type separation unit.

Aspect 4. The apparatus according to aspect 2 or aspect 3 wherein thesource of the second gas (17) comprises the first outlet of the secondseparation unit (20).

Aspect 5. The apparatus according to aspect 4 further comprising

-   -   a flow regulator (27) operatively disposed between the second        inlet of the gas mixer (60) and the first outlet of the second        separation unit (20) and in signal communication with the        controller (80);    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the flow        regulator (27) operatively disposed between the second inlet of        the gas mixture (60) and the first outlet of the second        separation unit (20).

Aspect 6. The apparatus according to any one of aspects 2 to 5 whereinthe source of the second gas (17) comprises the second outlet of thesecond separation unit (20).

Aspect 7. The apparatus according to aspect 6 further comprising

-   -   a flow regulator (29) operatively disposed between the second        inlet of the gas mixer (60) and the second outlet of the second        separation unit (20) and in signal communication with the        controller (80);    -   wherein the controller (80) is operable to control the flow rate        of the light noble gas from the source of the second gas (17) to        the second inlet of the gas mixer (60) by adjusting the flow        regulator (29) operatively disposed between the second inlet of        the gas mixer (60) and the second outlet of the second        separation unit (20).

Aspect 8. The apparatus according to any one of aspects 2 to 7

-   -   wherein the source of the second gas (17) comprises a process        gas transfer line (36) which operatively connects the product        gas header (210) to the inlet to the second separation unit        (20).

Aspect 9. The apparatus according to aspect 8 further comprising

-   -   a flow regulator (33) operatively disposed between the second        inlet of the gas mixer (60) and the process gas transfer line        (36) which operatively connects the product gas header (210) of        the adsorption separation unit (10) to the inlet of the second        separation unit (20) and in signal communication with the        controller (80);    -   wherein the controller (80) is operable to control the flow rate        of the light noble gas from the source of the second gas (17) to        the second inlet of the gas mixer (60) by adjusting the flow        regulator (33) operatively disposed between the second inlet of        the gas mixer (60) and the process gas transfer line which        operatively connects the product gas header (210) of the        adsorption separation unit (10) to the inlet of the second        separation unit (20).

Aspect 10. The apparatus according to any one of aspects 2 to 9

-   -   wherein the gas mixer (60) has a third inlet in fluid        communication with the second outlet of the second separation        unit (20).

Aspect 11. The apparatus according to aspect 10 further comprising

-   -   a flow regulator (31) operatively disposed between the third        inlet of the gas mixer (60) and the second outlet of the second        separation unit (20) and in signal communication with the        controller (80);    -   wherein the controller (80) is operable to adjust the flow        regulator (31) operatively disposed between the third inlet of        the gas mixer (60) and the second outlet of the second        separation unit (20) responsive to signals from the sensor (50).

Aspect 12. The apparatus according to any one of aspects 2 to 9

-   -   wherein the gas mixer (60) has a third inlet in fluid        communication with a process gas transfer line (36) which        operatively connects the product gas header (210) of the        adsorption separation unit (10) to the inlet to the second        separation unit (20).

Aspect 13. The apparatus according to aspect 12 further comprising

-   -   a flow regulator (37) operatively disposed between the third        inlet of the gas mixer (60) and the process gas transfer line        (36) which operatively connects the product gas header (210) of        the adsorption separation unit (10) to the inlet to the second        separation unit (20) and in signal communication with the        controller (80);    -   wherein the controller (80) is operable to adjust the flow        regulator (37) operatively disposed between the third inlet of        the gas mixer (60) and the process gas transfer line which        operatively connects the product gas header (210) of the        adsorption separation unit (10) to the inlet to the second        separation unit (20) responsive to signals from the sensor (50).

Aspect 14. The apparatus according to any one of aspects 2 to 13 whereinthe second separation unit (20) is a membrane-type separation unit,

-   -   wherein the source of the second gas (17) comprises the first        outlet of the second separation unit (20);    -   wherein the second separation unit (20) comprises one or more        adjustable orifices (26) in signal communication with the        controller (80), the one or more adjustable orifices (26)        operative to control a pressure in the second separation unit        (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the one or more        adjustable orifices (26).

Aspect 15. The apparatus according to any one of aspects 2 to 14 whereinthe second separation unit (20) is a membrane-type separation unit,

-   -   wherein the source of the second gas (17) comprises the first        outlet of the second separation unit (20);    -   wherein the membrane-type separation unit comprises a plurality        of membrane modules and one or more control valves that control        the fraction of membrane modules on-stream, the one or more        control valves in signal communication with the controller (80);    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the fraction of        membrane modules on-stream.

Aspect 16. The apparatus according to any one of aspects 2 to 15 whereinthe second separation unit (20) is a membrane-type separation unit,wherein the source of the second gas (17) comprises the first outlet ofthe second separation unit (20), the apparatus further comprising

-   -   a heat exchanger (40) operative to control a temperature in the        second separation unit, the heat exchanger in signal        communication with the controller (80);    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the heat duty of        the heat exchanger (40).

Aspect 17. The apparatus according to any one of aspects 2 to 13 whereinthe second separation unit (20) is an adsorption-type separation unit,

-   -   wherein the adsorption-type separation unit comprises a        plurality of vessels each containing a bed of adsorbent, and one        or more control valves that control the fraction of the        plurality of vessels on-stream, the one or more control valves        in signal communication with the controller (80);    -   wherein the source of the second gas comprises the first outlet        of the second separation unit;    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the fraction of        the plurality of vessels on-stream.

Aspect 18. The apparatus according to any one of aspects 2 to 13 oraspect 17 wherein the second separation unit (20) is an adsorption-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit;    -   wherein the second separation unit (20) comprises a feed gas        header,    -   wherein the second separation unit (20) comprises one or more        adjustable orifices (32) operative to control a pressure in the        feed gas header of the second separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the one or more        adjustable orifices (32) operative to control the pressure in        the feed gas header of the second separation unit (20).

Aspect 19. The apparatus according to any one of aspects 2 to 13 or 17to 18 wherein the second separation unit (20) is an adsorption-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit;    -   wherein the second separation unit (20) comprises a tail gas        header,    -   wherein the second separation unit (20) comprises one or more        adjustable orifices (27) operative to control a pressure in the        tail gas header of the second separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the one or more        adjustable orifices (27) operative to control the pressure in        the tail gas header of the second separation unit (20).

Aspect 20. The apparatus according to any one of aspects 2 to 13 or 17to 18 wherein the second separation unit (20) is an adsorption-typeseparation unit,

-   -   wherein the source of the second gas comprises the second outlet        of the second separation unit;    -   wherein the second separation unit (20) comprises a product gas        header,    -   wherein the second separation unit (20) comprises one or more        adjustable orifices (26) operative to control a pressure in the        product gas header of the second separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the one or more        adjustable orifices (26) operative to control the pressure in        the product gas header of the second separation unit (20).

Aspect 21. The apparatus according to any one of aspects 2 to 13 whereinthe second separation unit (20) is a distillation-type separation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit (20);    -   wherein the second separation unit comprises one or more        adjustable orifices (26, 27, 32) in signal communication with        the controller (80), the one or more adjustable orifices (26,        27, 32) operative to control a pressure in the second separation        unit (20);    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the one or more        adjustable orifices (26, 27, 32) operative to control the        pressure in the second separation unit (20).

Aspect 22. The apparatus according to any one of aspects 2 to 13 or 21wherein the second separation unit (20) is a distillation-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit (20);    -   the apparatus further comprising a heat exchanger (40) operative        to control a temperature in the second separation unit, the heat        exchanger in signal communication with the controller (80);    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the heat duty of        the heat exchanger (40).

Aspect 23. The apparatus according to any one of aspects 2 to 13 or 21to 22 wherein the second separation unit (20) is a distillation-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit (20);    -   wherein the second separation unit (20) comprises one or more        orifices operative to control a reflux ratio in the second        separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the reflux ratio        in the second separation unit (20).

Aspect 24. The apparatus according to any one of aspects 2 to 13 or 21to 23 wherein the second separation unit (20) is a distillation-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit (20);    -   wherein the second separation unit (20) comprises one or more        orifices operative to control a distillate to feed ratio in the        second separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the distillate        to feed ratio in the second separation unit (20).

Aspect 25. The apparatus according to any one of aspects 2 to 13 or 21to 24 wherein the second separation unit (20) is a distillation-typeseparation unit,

-   -   wherein the source of the second gas comprises the first outlet        of the second separation unit (20);    -   wherein the second separation unit (20) comprises one or more        orifices operative to control a product to feed ratio in the        second separation unit (20); and    -   wherein the controller (80) is operable to control the flow rate        of light noble gas from the source of the second gas (17) to the        second inlet of the gas mixer (60) by adjusting the distillate        to feed ratio in the second separation unit (20).

Aspect 26. A process for separating a feed gas stream (11) comprising alight noble gas and at least one other gaseous component into a lightnoble gas-rich stream (25) and a light noble gas-lean stream (14), thelight noble gas selected from the group consisting of helium, neon, andargon, the process comprising:

-   -   combining the feed gas stream (11) with a second gas stream (17)        to form a combined gas stream (12), the second gas stream (17)        having higher light noble gas content than the feed gas stream        (11), the second gas stream (17) having a flow rate that is        regulated;    -   separating a first separation unit feed gas stream (15) in an        adsorption separation unit (10) to produce a light noble        gas-enriched stream (13) and the light noble gas-lean stream        (14), wherein the first separation unit feed gas stream (15)        comprises at least a portion of the combined gas stream (12);        and    -   separating a second separation unit feed gas stream (21) in a        second separation unit (20) to produce the light noble gas-rich        stream (25) and a light noble gas-depleted stream (23), wherein        said second separation unit feed gas stream (21) comprises at        least a portion of the light noble gas-enriched stream (13) from        the adsorption separation unit (10);    -   wherein the flow rate of the light noble gas in the second gas        stream (17) is controlled responsive to a measure of the light        noble gas content in at least one of the feed gas stream (11),        the combined gas stream (12), or the first separation unit feed        gas stream (15).

Aspect 27. The process according to aspect 26 wherein the adsorptionseparation unit (10) comprises:

-   -   a plurality of vessels (100 a, 100 b, 100 c, 100 d, 100 e) each        containing a bed of adsorbent;    -   a feed gas header (200) in selective fluid communication with        each of the plurality of vessels (100 a, 100 b, 100 c, 100 d,        100 e);    -   a product gas header (210) in selective fluid communication with        each of the plurality of vessels (100 a, 100 b, 100 c, 100 d,        100 e);    -   a tail gas header (220) in selective fluid communication with        each of the plurality of vessels (100 a, 100 b, 100 c, 100 d,        100 e);    -   process gas transfer lines operatively connecting the plurality        of vessels (100 a, 100 b, 100 c, 100 d, 100 e) to the feed gas        header (200), the product gas header (210), and the tail gas        header (220);    -   each vessel (100) of the plurality of vessels (100 a, 100 b, 100        c, 100 d, 100 e) having process gas transfer lines associated        therewith (101, 102, 103, 104, 105, 106, 107, 108);    -   a plurality of valves in the process gas transfer lines        including a plurality of valves adjacent and associated (110,        111, 112, 113, 114, 115) with each respective vessel (100);    -   wherein the adsorption separation unit (10) has a central        volume, V_(c), of process gas transfer lines (101, 102, 103,        104, 105, 106, 107, 108) associated with each of the respective        vessels (100),    -   wherein the central volume for each respective vessel is the sum        of        -   (i) the volume contained in the process gas transfer lines            associated with the respective vessel connecting the            respective vessel to each valve adjacent (110, 111, 112,            113, 114, 115) the respective vessel (100),        -   (ii) all dead-end volumes (109), if any, connected at a            junction to the respective vessel (100), and        -   (iii) all dead-end volumes, if any, connected at a junction            to any of the process gas transfer lines associated with the            respective vessel (100) that connect the respective vessel            (100) to any valve adjacent (110, 111, 112, 113, 114, 115)            the respective vessel (100);    -   wherein the central volume for each respective vessel includes a        secondary volume, V₂, where the secondary volume is the sum of        -   (i) the volume of all dead-end volumes (109), if any,            connected to the respective vessel (100);        -   (ii) the volume of all dead-end volumes, if any, connected            at a junction to any of the process gas transfer lines            associated with the respective vessel (100) that connect the            respective vessel (100) to any valve adjacent (110, 111,            112, 113, 114, 115) the respective vessel (100), and        -   (iii) the volume of any process gas transfer lines (108), if            any, having a first end terminating in a valve adjacent            (115) the respective vessel (100) that is configured to            permit transfer of process gas to the tail gas header (220)            when open and having a second end terminating at a junction            to any other of the associated process gas transfer lines            (102) that connect the respective vessel (100) to any other            valve adjacent (110) the respective vessel (100); and    -   wherein the secondary volume, V₂, is less than 5% or less than        3% or less than 1% of the central volume, V_(c), for each vessel        (100).

Aspect 28. The process according to aspect 26 or aspect 27 where thefeed gas stream (11) has a total gas molar flow rate, F₁, with a molarflow rate of light noble gas, F_(1, Noble), and the second gas stream(17) has a total gas molar flow rate, F₂, with a molar flow rate oflight noble gas, F_(2, Noble), and wherein

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}} \geq 1.$

Aspect 29. The process according to any one of aspects 26 to 28 whereinthe second separation unit (20) is an adsorption-type separation unit, amembrane-type separation unit, or a distillation-type separation unit.

Aspect 30. The process according to any one of aspects 26 to 29

-   -   wherein the flow rate of light noble gas in the second gas        stream (17) is increased if the light noble gas content is less        than a desired lower limit; and/or    -   wherein the flow rate of light noble gas in the second gas        stream (17) is decreased if the light noble gas content is        greater than a desired upper limit.

Aspect 31. The process according to any one of aspects 26 to 30 whereinthe second gas stream (17) comprises the light noble gas-depleted stream(23), and wherein the flow rate of light noble gas in the second stream(17) is increased or decreased by controlling operating conditions ofthe second separation unit (20) in response to the light noble gascontent.

Aspect 32. The process according to aspect 31 wherein the secondseparation unit (20) is a membrane-type separation unit and whereincontrolling operating conditions of the second separation unit (20)comprises

-   -   decreasing the pressure difference between the second separation        unit feed gas stream (21) and the light noble gas-rich stream        (25) to increase the flow rate of light noble gas in the light        noble gas-depleted stream (23); and/or    -   increasing the pressure difference between the second separation        unit feed gas stream (21) and the light noble gas-rich stream        (25) to decrease the flow rate of light noble gas in the light        noble gas-depleted stream (23).

Aspect 33. The process according to aspect 31 or aspect 32 wherein thesecond separation unit (20) is a membrane-type separation unitcomprising a plurality of membrane modules, and wherein controllingoperating conditions of the second separation unit (20) comprises

-   -   decreasing the number of membrane modules on-stream to increase        the flow rate of light noble gas in the light noble gas-depleted        stream (23); and/or    -   increasing the number of membrane modules on-stream to decrease        the flow rate of light noble gas in the light noble gas-depleted        stream (23).

Aspect 34. The process according to any one of aspects 31 to 33 whereinthe second separation unit (20) is a membrane-type separation unit, andwherein controlling operating conditions of the second separation unit(20) comprises

-   -   increasing the temperature of the second separation unit feed        gas stream (21) to decrease the flow rate of light noble gas in        the light noble gas-depleted stream (23); and/or    -   decreasing the temperature of the second separation unit feed        gas stream (21) to increase the flow rate of light noble gas in        the light noble gas-depleted stream (23).

Aspect 35. The process according to aspect 31 wherein the secondseparation unit (20) is an adsorption-type separation unit operatingwith an adsorption cycle having a cycle time, and wherein controllingoperating conditions of the second separation unit (20) comprises

-   -   increasing the cycle time of the second separation unit (20) to        decrease the flow rate of light noble gas in the light noble        gas-depleted stream (23); and/or    -   decreasing the cycle time of the second separation unit (20) to        increase the flow rate of light noble gas in the light noble        gas-depleted stream (23).

Aspect 36. The process according to aspect 31 or 35 wherein the secondseparation unit (20) is an adsorption-type separation unit having a feedgas header and wherein controlling operating conditions of the secondseparation unit (20) comprises

-   -   increasing the pressure of the second separation unit feed gas        stream (21) in the feed gas header of the second separation unit        (20) to decrease the flow rate of the light noble gas in the        light noble gas-depleted stream (23); and/or    -   decreasing the pressure of the second separation unit feed gas        stream (21) in the feed gas header of the second separation unit        (20) to increase the flow rate of the light noble gas in the        light noble gas-depleted stream (23).

Aspect 37. The process according to any one of aspects 31, 35, or 36wherein the second separation unit (20) is an adsorption-type separationunit having a tail gas header and wherein controlling operatingconditions of the second separation unit (20) comprises

-   -   increasing the pressure of the light noble gas-depleted stream        (23) in the tail gas header of the second separation unit (20)        to increase the flow rate of the light noble gas in the light        noble gas-depleted stream (23); and/or    -   decreasing the pressure of the light noble gas-depleted stream        (23) in the tail gas header of the second separation unit (20)        to decrease the flow rate of the light noble gas in the light        noble gas-depleted stream (23).

Aspect 38. The process according to any one of aspects 31, or 35 to 37wherein the second separation unit (20) is an adsorption-type separationunit operating with an adsorption cycle comprising a blowdown stephaving a target pressure for the end of the blowdown step, where ablowdown gas stream is formed during the blowdown step and whereincontrolling operating conditions of the second separation unit (20)comprises

-   -   increasing the target pressure for the end of the blowdown step        to increase the flow rate of the light noble gas in the light        noble gas-depleted stream (23); and/or    -   decreasing the target pressure for the end of the blowdown step        to decrease the flow rate of the light noble gas in the light        noble gas-depleted stream (23).

Aspect 39. The process according to any one of aspects 31, or 35 to 38wherein the second separation unit (20) is an adsorption-type separationunit comprising a plurality of adsorption beds and operating with aplurality of adsorption cycles each comprising a feed step, whereincontrolling the operating conditions of the second separation unit (20)comprises:

-   -   changing to an adsorption cycle having fewer adsorption beds        simultaneously on the feed step to increase the flow rate of        light noble gas in the light noble gas-depleted stream (23);        and/or    -   changing to an adsorption cycle having a greater number of        adsorption beds simultaneously on the feed step to increase the        flow rate of light noble gas in the light noble gas-depleted        stream (23).

Aspect 40. The process according to any one of aspects 31, or 35 to 39wherein the second separation unit (20) is an adsorption-type separationunit comprising a plurality of adsorption beds and operating with aplurality of adsorption cycles, some comprising a pressure equalizationstep, wherein controlling the operating conditions of the secondseparation unit (20) comprises:

-   -   changing to an adsorption cycle having fewer or no pressure        equalization steps to increase the flow rate of light noble gas        in the light noble gas-depleted stream (23); and/or    -   changing to an adsorption cycle having a greater number of        pressure equalization steps to decrease the flow rate of light        noble gas in the light noble gas-depleted stream (23).

Aspect 41. The process according to aspect 31 wherein the secondseparation unit (20) is a distillation-type separation unit having anoperating pressure wherein controlling the operating conditions of thesecond separation unit (20) comprises:

-   -   decreasing the operating pressure of the second separation unit        (20) to increase the flow rate of light noble gas in the light        noble gas-depleted stream (23); and/or    -   increasing the operating pressure of the second separation unit        (20) to decrease the flow rate of light noble gas in the light        noble gas-depleted stream (23).

Aspect 42. The process according to aspect 31 or aspect 41 wherein thesecond separation unit (20) is a distillation-type separation unitoperating with a reflux ratio wherein controlling the operatingconditions of the second separation unit (20) comprises:

-   -   increasing the reflux ratio of the second separation unit (20)        to increase the flow rate of light noble gas in the light noble        gas-depleted stream (23); and/or    -   decreasing the reflux ratio of the second separation unit (20)        to decrease the flow rate of light noble gas in the light noble        gas-depleted stream (23).

Aspect 43. The process according to any one of aspects 31 or 41 to 42wherein the second separation unit (20) is a distillation-typeseparation unit having an operating temperature wherein controlling theoperating conditions of the second separation unit (20) comprises:

-   -   decreasing the operating temperature of the second separation        unit (20) to increase the flow rate of light noble gas in the        light noble gas-depleted stream (23); and/or    -   increasing the operating temperature of the second separation        unit (20) to decrease the flow rate of light noble gas in the        light noble gas-depleted stream (23).

Aspect 44. The process according to any one of aspects 26 to 43 whereinthe second gas stream (17) comprises a portion (28) of the light noblegas-rich stream (25) having a flow rate, and the flow rate of the lightnoble gas in the second gas steam (17) is increased by increasing theflow rate of the portion (28) of the light noble gas-rich stream (25)and decreased by decreasing the flow rate of the portion (28) of thelight noble gas-rich stream (25).

Aspect 45. The process according to any one of aspects 26 to 44 whereinthe feed gas stream (11) has a molar concentration of light noble gasranging from 0.1 mole % to 2.0 mole %, or ranging from 0.1 mole % to 1.0mole %.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a process flow diagram for a light noble gas recovery processaccording to the present process and apparatus.

FIG. 2 is a process flow diagram for an adsorption separation unitsuitable for the process.

FIG. 3 is a cycle chart for a 5-bed adsorption separation unit suitablefor the process.

FIG. 4 is a plot of helium recovery versus % helium in the feed gas.

FIG. 5 is a plot of helium recovery versus helium content in the productstream.

FIG. 6 is a plot of helium content in the product stream versus mole %helium in the feed gas stream.

FIG. 7 shows plots of helium recovery versus % helium in the feed gasstream.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ensuing detailed description provides preferred exemplaryembodiments only, and is not intended to limit the scope, applicability,or configuration of the invention. Rather, the ensuing detaileddescription of the preferred exemplary embodiments will provide thoseskilled in the art with an enabling description for implementing thepreferred exemplary embodiments of the invention, it being understoodthat various changes may be made in the function and arrangement ofelements without departing from the scope of the invention as defined bythe claims.

The articles “a” or “an” as used herein mean one or more when applied toany feature in embodiments of the present invention described in thespecification and claims. The use of “a” and “an” does not limit themeaning to a single feature unless such a limit is specifically stated.The article “the” preceding singular or plural nouns or noun phrasesdenotes a particular specified feature or particular specified featuresand may have a singular or plural connotation depending upon the contextin which it is used.

The adjective “any” means one, some, or all, indiscriminately ofquantity.

The term “and/or” placed between a first entity and a second entityincludes any of the meanings of (1) only the first entity, (2) only thesecond entity, and (3) the first entity and the second entity. The term“and/or” placed between the last two entities of a list of 3 or moreentities means at least one of the entities in the list including anyspecific combination of entities in this list. For example, “A, B and/orC” has the same meaning as “A and/or B and/or C” and comprises thefollowing combinations of A, B and C: (1) only A, (2) only B, (3) onlyC, (4) A and B and not C, (5) A and C and not B, (6) B and C and not A,and (7) A and B and C.

The phrase “at least one of” preceding a list of features or entitiesmeans one or more of the features or entities in the list of entities,but not necessarily including at least one of each and every entityspecifically listed within the list of entities and not excluding anycombinations of entities in the list of entities. For example, “at leastone of A, B, or C” (or equivalently “at least one of A, B, and C” orequivalently “at least one of A, B, and/or C”) has the same meaning as“A and/or B and/or C” and comprises the following combinations of A, Band C: (1) only A, (2) only B, (3) only C, (4) A and B and not C, (5) Aand C and not B, (6) B and C and not A, and (7) A and B and C.

The term “plurality” means “two or more than two.”

The phrase “at least a portion” means “a portion or all.” The at least aportion of a stream may have the same composition, with the sameconcentration of each of the species, as the stream from which it isderived. The at least a portion of a stream may have a differentconcentration of species than that of the stream from which it isderived. The at least a portion of a stream may include only specificspecies of the stream from which it is derived.

As used herein a “divided portion” of a stream is a portion having thesame chemical composition and species concentrations as the stream fromwhich it was taken.

As used herein a “separated portion” of a stream is a portion having adifferent chemical composition and different species concentrations thanthe stream from which it was taken. A separated portion may be, forexample, a portion formed from separation in a separator.

The term “portion” includes both a “divided portion” and a “separatedportion.”

As used herein, “first,” “second,” “third,” etc. are used to distinguishamong a plurality of steps and/or features, and is not indicative of thetotal number, or relative position in time and/or space, unlessexpressly stated as such.

The terms “depleted” or “lean” mean having a lesser mole % concentrationof the indicated component than the original stream from which it wasformed. “Depleted” and “lean” do not mean that the stream is completelylacking the indicated component.

The terms “rich” or “enriched” mean having a greater mole %concentration of the indicated component than the original stream fromwhich it was formed.

As used herein, “in fluid communication” or “in fluid flowcommunication” means operatively connected by one or more conduits,manifolds, valves and the like, for transfer of fluid. A conduit is anypipe, tube, passageway or the like, through which a fluid may beconveyed. An intermediate device, such as a pump, compressor or vesselmay be present between a first device in fluid flow communication with asecond device unless explicitly stated otherwise.

Downstream and upstream refer to the intended flow direction of theprocess fluid transferred. If the intended flow direction of the processfluid is from the first device to the second device, the second deviceis downstream of the first device. In case of a recycle stream,downstream and upstream refer to the first pass of the process fluid.

For the purposes of simplicity and clarity, detailed descriptions ofwell-known devices, circuits, and methods are omitted so as not toobscure the description of the present invention with unnecessarydetail.

The present apparatus and process are described with reference to thefigures. In this disclosure, a single reference number may be used toidentify a process gas stream and the process gas transfer line thatcarries said process gas stream. Which feature the reference numberrefers to will be understood depending on the context.

The present apparatus and process are for separating a feed gas 11containing a light noble gas and at least one other gaseous componentinto a light noble gas-rich gas 25 and a light noble gas-lean gas 14.The light noble gas may be helium, neon, or argon.

The feed gas 11 may be natural gas. The light noble gas may be helium.The at least one other component may be methane. Another component maybe nitrogen. Some natural gas sources are known to comprise methane,nitrogen, and helium.

The feed gas 11 may be a non-condensable vent gas from an air separationunit (ASU). The light noble gas may be neon. The at least one othercomponent may be nitrogen. Another component may be oxygen. Anothercomponent may be argon. Air is known to comprise nitrogen, oxygen,argon, and neon.

The feed gas 11 may be natural gas. The light noble gas may be argon.The at least one other component may be methane. Another component maybe carbon dioxide. Another component may be nitrogen. Some natural gassources are known to comprise methane, carbon dioxide, nitrogen, andargon.

The apparatus comprises an adsorption separation unit 10. Anadsorption-type separation unit is any separation unit that separates afeed stream into at least two streams using a solid adsorbent, onestream enriched in species that are more adsorbable and another streamenriched in species that are less adsorbable. The adsorption separationunit 10 comprises a plurality of vessels 100 a, 100 b, 100 c, 100 d, 100e. Each of the plurality of vessels contains a bed of adsorbent suitablefor separating the light noble gas from the other components in the feedstream.

Adsorption separation units generally comprise a plurality of adsorptionbeds containing suitable adsorbent. While FIGS. 1-3 provide an exemplaryadsorption unit with five adsorption beds, any suitable number ofadsorption beds may be used. In general, the number of adsorption bedsused in the adsorption separation unit and process is designed to meetrequired product purity and light noble gas product recovery.

For a required product purity, the number of beds can be a trade-offbetween capital and light noble gas recovery. For example, increasingthe number of beds allows the adsorption process to utilize a greaternumber of pressure equalization steps. Pressure equalization steps arelight noble gas saving steps. Increasing the number of pressureequalization steps will reduce the pressure at which gas is dischargedfrom the bed to the waste stream, decreasing light noble gas losses. Ifthe pressure equalization steps are conducted through co-currentdepressurization of the high-pressure bed, the impurity front advancesfarther when more pressure equalization steps are used. To maintain thedesired production, the size of each bed increases in addition to thenumber of beds.

Alternatively, the number of beds may be increased to lengthen the timeavailable to individual steps that may be limiting the efficiency of theoverall process. For example, increasing the number of beds allows theadsorption process to increase the number of beds that will process feedgas or process purge gas. Sending gas to more beds on feed or more bedson purge decreases the velocity of the gas passing over the adsorbentparticles, which in turn increases the efficiency of the process step.

Generally, more than one adsorption bed is used so that at least oneadsorption bed can be producing product gas while another bed isregenerating. In this way, product gas can be produced on a continuousbasis.

FIG. 1 illustrates a 5-bed adsorption separation unit with beds A-E.FIG. 2 illustrates adsorption separation unit 10 with 5 adsorptionvessels 100 a, 100 b, 100 c, 100 d, and 100 e. The skilled person canreadily select the number of adsorption vessels/beds to use.

The adsorption beds may contain a single adsorbent or multipleadsorbents. In the case of multiple adsorbents, the adsorbents may beinterspersed, layered, or a combination thereof.

Suitable adsorbents may be readily selected by those skilled in the art.Suitable adsorbents for separating helium from other gaseous componentsin natural gas include activated carbon, silica gel, activated alumina,and zeolites such as molecular sieves.

The adsorption separation unit may be operated using any knownadsorption cycle suitable for separating a light noble gas from a gasmixture comprising the light noble gas and at least one other gascomponent. Examples are illustrated in U.S. Pat. No. 4,077,779A and U.S.Pat. No. 774,068B2, which both detail pressure swing adsorption cyclesfor light gas purification.

The adsorption cycle may be a so-called vacuum pressure swing adsorption(VPSA) cycle.

The adsorption cycle may comprise a production step, a co-current rinsestep, a blowdown step, an evacuation step, and a product pressurizationstep. In the exemplary embodiment shown in FIG. 1, adsorption beds A-Eare illustrated with a cycle having a production step (P), a co-currentrinse step (R), a blowdown step (BD), an evacuation step (EV), and aproduct pressurization step (PP). During the adsorption cycle, each ofthe beds cycle in turn through the cycle steps. The corresponding VPSAcycle table for a 5-bed adsorption separation unit is shown in FIG. 3.

The production step is abbreviated herein as “P”. The production step isalso called the feed step and/or adsorption step in the literature.

As shown in FIG. 2, the adsorption separation unit 10 comprises a feedgas header 200. The feed gas header 200 is in selective fluidcommunication with each of the plurality of vessels 100 a, 100 b, 100 c,100 d, and 100 e for providing a respective portion of separation unitfeed gas stream 12 to each of the plurality of vessels. “Selective”fluid communication means that a valve or equivalent device is used toselectively provide fluid communication between components (i.e. betweenthe feed gas header and each of the plurality of vessels). As shown inFIG. 2, valve 110 a provides selective fluid communication between thefeed gas header 200 and adsorption vessel 100 a, valve 110 b providesselective fluid communication between the feed gas header 200 andadsorption vessel 100 b, and so on.

In the embodiment shown in FIGS. 1 and 3, the adsorption vessel feed gas15 comprises the separation unit feed gas stream 12 and rinse gaseffluent 18 from an adsorption vessel undergoing a rinse step. In theembodiment shown in FIG. 2, the feed gas header 200 receives theseparation unit feed gas stream 12 and rinse gas effluent 18 from anadsorption vessel undergoing a rinse step via the rinse gas effluentheader 250. A rinse gas effluent header 250 may or may not be useddepending on the selected adsorption cycle. The separation unit feed gasstream 12 is formed from the feed gas stream 11 and second gas stream 17(discussed in more detail below).

Adsorption vessel feed gas 15 passes from the feed gas header 200,through a respective open feed gas valve 110 (valve 110 a for vessel 100a, valve 110 b for vessel 100 b, etc.), and into a respective adsorptionvessel 100 for a production step of the adsorption cycle.

In the production step, the adsorption vessel feed gas stream 15containing the light noble gas (for example helium) is introduced at afeed gas pressure into an adsorption bed undergoing the production stepand the secondary gas components (for example CH₄ and N₂) are adsorbedon the adsorbent in the adsorption bed undergoing the production stepwhile a light noble gas-enriched stream 13 is simultaneously withdrawnfrom the adsorption bed undergoing the production step and passed to aproduct gas header 210. The product gas header 210 is in selective fluidcommunication with each of the plurality of vessels 100 a, 100 b, 100 c,100 d, and 100 e for receiving light noble gas-enriched gas 13 from eachof the plurality of vessels when on a production step. The product gasheader 210 is in selective fluid communication with each respectivevessel 100 by way of a respective product valve 113 (valve 113 a forvessel 100 a, valve 113 b for vessel 100 b, etc.). The light noblegas-enriched gas 13 contains a higher concentration of light noble gasthan the adsorption vessel feed gas stream 15 and is depleted of thesecondary gas components. The duration of the production step may be anysuitable duration, for example from 1 second to 300 seconds, or from 30seconds to 300 seconds. The skilled person can readily determine asuitable duration for any of the known adsorption cycle steps.

The pressure in the adsorption bed undergoing the production step mayrange, for example from 0.1 MPa to 3.4 MPa or from 0.3 MPa to 1.2 MPa(absolute pressure).

Each of the adsorption beds has a “feed end” and a “product end,” sotermed because of their function during the production step of theadsorption cycle. A feed gas mixture is introduced into the “feed end”of the adsorption bed and a product gas is withdrawn from the “productend” during the production step of the cycle. During other steps of theadsorption cycle, gas may be introduced or withdrawn from “feed end.”Likewise, during other steps of the adsorption cycle, gas may beintroduced or withdrawn from the “product end.”

The direction of flow during other steps is typically described withreference to the direction of flow during the production step. Thus, gasflow in the same direction as the gas flow during the production step is“co-current” (sometimes called “concurrent”) and gas flow that is in theopposite direction to the gas flow during the production step is“counter-current.” Co-currently introducing a gas into an adsorption bedmeans to introduce the gas in the same direction as the feed gasintroduced during the production step (i.e. introducing into the feedend). Counter-currently introducing a gas into an adsorption bed meansto introduce the gas in a direction opposite to the direction of thefeed gas flow during the feed step (i.e. introducing into the productend). Co-currently withdrawing a gas from an adsorption bed means towithdraw the gas in the same direction as the product gas during theproduction step (i.e. withdrawing from the product end).Counter-currently withdrawing a gas from an adsorption bed means towithdraw the gas in a direction opposite to the direction of the productgas flow during the production step (i.e. withdrawing from the feedend).

The blowdown step, abbreviated “BD”, comprises counter-currentlywithdrawing a blowdown gas from an adsorption bed undergoing theblowdown step. The blowdown gas has a concentration of the secondary gascomponents that is higher than the concentration of the secondary gascomponents in the adsorption vessel feed gas stream 15. The blowdown gasmay be withdrawn from the adsorption bed undergoing the counter-currentblowdown step until the pressure in the adsorption bed undergoing thecounter-current blowdown step reaches a blowdown pressure ranging from40 kPa to 1000 kPa. The blowdown pressure is the pressure in theadsorption bed at the end of the counter-current blowdown step.

As shown in FIG. 1, the blowdown gas may be passed from the adsorptionbed undergoing the blowdown step to a buffer vessel 70 and compressed incompressor 56 to form a rinse gas 19 for the rinse step, “R”. As shownin FIG. 2, blowdown gas is passed to blowdown gas header 230, to buffervessel 70, to compressor 56, and passed to rinse gas supply header 240.A rinse gas supply header 240 may or may not be used depending on theselected adsorption cycle.

The rinse step is abbreviated “R”. The rinse step comprises co-currentlyintroducing a rinse gas 19 into an adsorption bed undergoing the rinsestep while simultaneously co-currently withdrawing a rinse gas effluent18 from the adsorption bed undergoing the rinse step. During a rinsestep, the more strongly adsorbed components displace the less stronglyadsorbed components from the adsorbent and void spaces, providing ameans to increase the recovery of the less strongly adsorbed components,i.e. the light noble gas. A rinse gas may be formed from the blowdowngas. A rinse gas may also be formed from the light noble gas-depletedstream, an external source of gas which is absent of light noble gas, orany combination thereof.

As shown in FIG. 1, the rinse gas effluent stream 18 may be introducedalong with the adsorption separation unit feed gas stream 12 into theadsorption bed undergoing the production step, “P”, as adsorption vesselfeed gas stream 15.

Forming a rinse gas from the blowdown gas, in combination with the rinsestep, in combination with introducing the rinse gas effluent into theadsorption bed undergoing the production step has the technical effectof increasing the recovery of light noble gas.

As shown in FIG. 1, the adsorption cycle may also include an evacuationstep, “EV”. The evacuation step is similar to the blowdown step, withthe addition of using a compressor, vacuum pump, or the like, 57 to drawthe pressure down below atmospheric pressure. The gas evacuated from theadsorption bed undergoing the evacuation step is passed to a tail gasheader 220, and then to a compressor 57 where it is discharged from thecompressor 57 as light noble gas-lean stream 14.

The tail gas header 220 is in selective fluid communication with each ofthe plurality of vessels for receiving light noble gas-lean gas fromeach of the plurality of vessels. Selective fluid communication betweenthe tail gas header and each of the respective adsorption vessels 100 isprovided via valves 115 and 116 (115 a and 116 a for vessel 100 a, 115 band 116 b for vessel 100 b, etc.). The tail gas header 220 is in fluidcommunication with an outlet of the adsorption separation unit 10 fordischarging the light noble gas-lean gas from the adsorption separationunit 10. For the case where the feed gas stream 11 is natural gas thatcontains helium, light noble gas-lean stream 14 is natural gas strippedof helium and can be introduced into a natural gas pipeline for anydesired use.

Depending on the adsorption cycle used, the blowdown gas header may alsobe a tail gas header, for example when the blowdown gas is dischargedfrom the adsorption separation unit 10 as a light noble gas-lean gas.

As shown in FIG. 1, the adsorption cycle may also include a productpressurization step, “PP”. The product pressurization step comprisescounter-currently introducing a portion of the product gas 16 into thebed to pressurize the vessel. As shown in FIG. 2, product gas 16 may beintroduced into a respective adsorption vessel 100 from product gasheader 210 via a respective product gas valve 113. Product gas 16 may beintroduced into the adsorption bed undergoing the product pressurizationstep until the adsorption bed undergoing the product pressurization stepis substantially at the feed gas pressure.

If desired, the adsorption cycle may include various other adsorptioncycle steps, such as pressure equalization steps. Various adsorptioncycle steps are discussed, for example, in U.S. Pat. No. 9,381,460.

Adsorption cycles have a cycle time. The cycle time is a well-understoodand conventional term in the art. The adsorption separation unitundergoes a repeated series of cycle steps of a defined adsorptioncycle. The cycle time is the time period required to complete oneadsorption cycle from start to finish.

The plurality of vessels 100 a, 100 b, 100 c, 100 d, 100 e areoperatively connected to the various headers 200, 210, 220, 230, 240,and 250 by respective process gas transfer lines 101, 102, 103, 104,105, 106, 107, and 108. As used herein, process gas transfer lines areany fluid-tight conveyance means for transferring process gas therein,for example, pipes, tubes, conduits, ducts, hoses, etc.

Each vessel of the plurality of vessels 100 a, 100 b, 100 c, 100 d, 100e have process gas transfer lines associated therewith (process gastransfer lines 101 a, 102 a, 103 a, 104 a, 105 a, 106 a, 107 a, and 108a for vessel 100 a; process gas transfer lines 101 b, 102 b, 103 b, 104b, 105 b, 106 b, 107 b, and 108 b for vessel 100 b; process gas transferlines 101 c, 102 c, 103 c, 104 c, 105 c, 106 c, 107 c, and 108 c forvessel 100 c; process gas transfer lines 101 d, 102 d, 103 d, 104 d, 105d, 106 d, 107 d, and 108 d for vessel 100 d; and process gas transferlines 101 e, 102 e, 103 e, 104 e, 105 e, 106 e, 107 e, and 108 e forvessel 100 e). Process gas transfer lines are “associated” with aspecific vessel if they provide fluid communication between the specificvessel and an adjacent header. Process gas transfer lines are associatedwith a specific vessel if they are operatively disposed between thespecific vessel and an adjacent header (and not beyond the adjacentheader). Referring to FIG. 2, only the “a” process gas transfer linesare associated with vessel 100 a, only the “b” process gas transferlines are associated with vessel 100 b, only the “c” process gastransfer lines are associated with vessel 100 c, only the “d” processgas transfer lines are associated with vessel 100 d, only the “e”process gas transfer lines are associated with vessel 100 e.

As shown in FIG. 2, there are a plurality of valves (valves 110 a, 111a, 112 a, 113 a, 114 a, 115 a, and 116 a for vessel 100 a; valves 110 b,111 b, 112 b, 113 b, 114 b, 115 b, and 116 b for vessel 100 b; valves110 c, 111 c, 112 c, 113 c, 114 c, 115 c, and 116 c for vessel 100 c;valves 110 d, 111 d, 112 d, 113 d, 114 d, 115 d, and 116 d for vessel100 d; valves 110 e, 111 e, 112 e, 113 e, 114 e, 115 e, and 116 e forvessel 100 e) in the process gas transfer lines including a plurality ofvalves adjacent and associated with each respective vessel 100. Thevalves control the flow of process gas to and from the adsorptionvessels 100 a, 100 b, 100 c, 100 d, and 100 e in order to implement thevarious cycle steps.

Valves are “associated” with a specific vessel if they are operativelydisposed between the specific vessel and an adjacent header (and notbeyond the adjacent header). A valve associated with a specific vesselcan control flow between the specific vessel and an adjacent header.

A valve is “adjacent” the vessel if no other valve is operativelydisposed in the process gas transfer line between said valve and thevessel; there is no valve interposed in the process gas transfer linebetween an adjacent valve and the respective vessel. A valve in aprocess gas transfer line operatively disposed between a second valveand the vessel, when closed, would prevent process gas flow to thevessel from the second valve or from the vessel to the second valve.

Referring to FIG. 2, valves 110 a, 111 a, 112 a, 113 a, 114 a, and 115 aare associated and adjacent to vessel 100 a. Valve 116 a is associatedwith vessel 100 a but not adjacent vessel 100 a because valve 115 a isoperatively disposed between vessel 100 a and valve 116 a. Likewise,valves 110 b, 111 b, 112 b, 113 b, 114 b, and 115 b are associated andadjacent to vessel 100 b. Valve 116 b is associated with vessel 100 bbut not adjacent vessel 100 b because valve 115 b is operativelydisposed between vessel 100 b and valve 116 b.

The present apparatus may be characterized by an adsorption separationunit 10 constructed to keep process gas transfer line volume anddead-end volumes that fill with substantial amounts light noble gasduring a pressurizing step and empty through the tail gas header duringa depressurizing step below a threshold level. These particular volumescarry a significant amount of light noble gas out of the system in astream that is not captured in the light noble gas product stream. Thatis, these volumes reduce the light noble gas recovery efficiency.

This separation unit construction may be defined in terms of a centralvolume and a secondary volume for each of the vessels 100 a, 100 b, 100c, 100 d, and 100 e, as detailed below.

The central volume for each vessel 100 is a specified volume of processgas transfer lines associated with each respective vessel 100. Thecentral volume for each respective vessel is the sum of:

-   -   (i) the volume contained in the process gas transfer lines        associated with the respective vessel connecting the respective        vessel to each valve adjacent 110, 111, 112, 113, 114, 115 the        respective vessel 100,    -   (ii) all dead-end volumes 109, if any, connected at a junction        to the respective vessel 100, and    -   (iii) all dead-end volumes (not shown), if any, connected at a        junction to any of the process gas transfer lines associated        with the respective vessel 100 that connect the respective        vessel 100 to any valve adjacent 110, 111, 112, 113, 114, 115        the respective vessel 100.

The central volume for a vessel 100 does not include the volume of thevessel itself.

A detailed description of the process gas transfer lines making up thecentral volume for vessel 100 a will be provided. The process gastransfer lines making up the central volume for vessels 100 b, 100 c,100 d, and 100 e are to be understood from this detailed descriptionwith the necessary changes having been made with regard to the referencenumbers (i.e. “b” substituted for “a” for vessel 100 b, “c” substitutedfor “a” for vessel 100 c, “d” substituted for “a” for vessel 100 d, and“e” substituted for “a” for vessel 100 e).

The central volume for vessel 100 a includes (i) the volume contained inthe process gas transfer lines associated with vessel 100 a connectingvessel 100 a to each valve adjacent vessel 100 a. With reference to FIG.2, the valves adjacent vessel 100 a include valves 110 a, 111 a, 112 a,113 a, 114 a, and 115 a. The process gas transfer lines associated withvessel 100 a and that connect vessel 100 a to these valves includeprocess gas transfer lines 102 a, 103 a, 104 a, 105 a, 107 a and 108 a.Process gas transfer line 102 a connects vessel 100 a to adjacent valve110 a. Process gas transfer line 103 a connects vessel 100 a to adjacentvalve 111 a. Process gas transfer line 104 a connects vessel 100 a toadjacent valve 112 a. Process gas transfer line 105 a connects vessel100 a to adjacent valve 113 a. Process gas transfer line 107 a connectsvessel 100 a to adjacent valve 114 a. Process gas transfer line 108 aconnects vessel 100 a to adjacent valve 115 a.

The central volume for vessel 100 a includes (ii) all dead-end volumes,if any, connected at a junction to the respective vessel 100.

A “dead-end volume” is defined as a volume in continuous open fluidcommunication with the respective vessel that permits ingress and egressof process gas only at the junction of the dead-end volume. “Continuousopen fluid communication” means that the dead-end volume is continuouslyin fluid communication with the respective vessel during the process. Novalve or other device cuts off fluid communication of the dead-endvolume from the respective vessel.

Referring to FIG. 2, process gas sensor line 109 a is such a dead-endvolume.

The adsorption separation unit 10 may be constructed so as to have nodead-end volumes connected at a junction to any of the vessels 100.

The central volume for vessel 100 a includes (iii) all dead-end volumes,if any, connected at a junction to any of the process gas transfer linesassociated with vessel 100 a that connects vessel 100 a to any valveadjacent 110 a, 111 a, 112 a, 113 a, 114 a, 115 a vessel 100 a. Theprocess gas transfer lines associated with vessel 100 a that connectvessel 100 a to adjacent valves 110 a, 111 a, 112 a, 113 a, 114 a, 115 ainclude process gas transfer lines 102 a, 103 a, 104 a, 105 a, 107 a,and 108 a. FIG. 2 shows no such dead-end volumes. However, one suchdead-end volume would exist if sensor line 109 a were moved to form ajunction with process gas transfer line 105 a instead of the junctionwith vessel 100 a.

The central volume for each vessel has a respective volume amount,V_(c).

The central volume for each respective vessel includes a secondaryvolume which is a subset of the central volume. The secondary volume isthe undesired volume that reduces the light noble gas recoveryefficiency.

The secondary volume for each respective vessel 100 is the sum of:

-   -   (i) the volume of all dead-end volumes, if any, connected to the        respective vessel 100,    -   (ii) the volume of all dead-end volumes, if any, connected at a        junction to any of the process gas transfer lines associated        with the respective vessel 100 that connect the respective        vessel 100 to any valve adjacent 110, 111, 112, 113, 114, 115        the respective vessel 100, and    -   (iii) the volume of any process gas transfer lines, if any,        having a first end terminating in a valve adjacent the        respective vessel 100 that is configured to permit transfer of        process gas to the tail gas header 220 when open and having a        second end terminating at a junction to any other of the        associated process gas transfer lines that connect the        respective vessel 100 to any other valve adjacent 110 the        respective vessel 100.

The secondary volume for a vessel 100 does not include the volume of thevessel itself.

A detailed description of the process gas transfer lines making up thesecondary volume for vessel 100 a will be provided. The process gastransfer lines making up the secondary volume for vessels 100 b, 100 c,100 d, and 100 e are to be understood from this detailed descriptionwith the necessary changes having been made with regard to the referencenumbers (i.e. “b” substituted for “a” for vessel 100 b, “c” substitutedfor “a” for vessel 100 c, “d” substituted for “a” for vessel 100 d, and“e” substituted for “a” for vessel 100 e).

The secondary volume for vessel 100 a includes (i) all dead-end volumes,if any, connected at a junction to the respective vessel 100.

Referring to FIG. 2, process gas sensor line 109 a is such a dead-endvolume according to criteria (i).

The adsorption separation unit 10 may be constructed so as to have nodead-end volumes connected at a junction to any of the vessels 100.

The secondary volume for vessel 100 a includes (ii) all dead-endvolumes, if any, connected at a junction to any of the process gastransfer lines associated with vessel 100 a that connects vessel 100 ato any valve adjacent 110 a, 111 a, 112 a, 113 a, 114 a, 115 a vessel100 a. The process gas transfer lines associated with vessel 100 a thatconnect vessel 100 a to adjacent valves 110 a, 111 a, 112 a, 113 a, 114a, 115 a include process gas transfer lines 102 a, 103 a, 104 a, 105 a,107 a, and 108 a. FIG. 2 shows no such dead-end volumes. However, onesuch dead-end volume would exist if sensor line 109 a were moved to forma junction with process gas transfer line 105 a instead of the junctionwith vessel 100 a.

The secondary volume for vessel 100 a includes (iii) the volume of anyprocess gas transfer lines, if any, having a first end terminating in avalve adjacent the respective vessel 100 a that is configured to permittransfer of process gas to the tail gas header 220 when open and havinga second end terminating at a junction to any other of the associatedprocess gas transfer lines that connect the respective vessel 100 a toany other valve adjacent the respective vessel 100 a. Referring to FIG.2, process gas transfer line 108 a has a first end terminating in valve115 a which is adjacent vessel 100 a. Valve 115 a is configured topermit transfer of process gas to the tail gas header 220 when open.Process gas transfer line 108 a has a second end terminating at ajunction with process gas transfer line 102 a which connects vessel 100a to valve 110 a which is adjacent vessel 100 a. Process gas transferline 108 a is therefore such a volume contributing to the secondaryvolume as per criteria (iii).

Each vessel has a respective secondary volume, V₂.

We have discovered that the secondary volume of an adsorption-typeseparation unit has an impact on the recovery efficiency of light noblegas where the feed gas composition has a low concentration of the lightnoble gas.

By providing an adsorption-type separation unit with a secondary volumewhich is much smaller than the central volume, more of the light noblegas can be fed to a second separation unit to meet the final productpurity specifications for the system.

It is desirable, therefore, to construct the adsorption separation unit10 with a secondary volume that is small relative to the central volume.The secondary volume, V₂, may be less than 5%, or less than 3%, or lessthan 1% of the central volume, V_(c), for each vessel 100. The secondaryvolume, V₂, may be 0.

Isolation valves (not shown) may be used to reduce the secondary volume.For example, with reference to vessel 100 a in FIG. 2, an isolationvalve (not shown) may be positioned in process gas transfer line 102 abetween vessel 100 a and valve 115 a. In this instance, valve 115 a isno longer an adjacent valve. The isolation valve can be operated toprevent process gas transfer line 108 a from filling with product gashaving a higher concentration of light noble gas duringrepressurization. Process gas transfer line 108 a may be insteadrepressurized in the rinse step with a rinse gas having a lowerconcentration of light noble gas. However, using isolation valves addsfurther complexity with regard to the timing of opening and closing theisolation valves during the adsorption cycle.

Also not shown are process gas transfer lines and valves associated witheach of the vessels for bypass, vent, startup, shutdown, maintenance andthe like, which are well-known and customary for adsorption separationunits. These process gas transfer lines may also contribute to thecentral volume and secondary volume.

The present apparatus may further comprise a second separation unit 20.The second separation unit may be an adsorption-type separation unit, amembrane-type separation unit, or a distillation-type separation unit.

An adsorption-type separation unit is any separation unit that separatesa feed stream into at least two streams using adsorption, each streamhaving a different concentration of species. As used herein, the term“adsorption” includes any separation which separates components by theirrelative adhesion of species (atoms, ions, or molecules) to an adsorbentsurface.

A membrane-type separation unit is any separation unit that separates afeed stream into two streams using a membrane, each stream having adifferent concentration of species. The membrane can be a semipermeablemembrane or a selectively permeable membrane. Membranes separate gasmixtures by allowing certain gas species to pass through the membrane bydiffusion, facilitated diffusion, passive transport, and/or activetransport.

A distillation-type separation unit is any separation unit thatseparates a feed stream into two streams using distillation, each streamhaving a different concentration of species. As used herein, the term“distillation” includes any separation which separates components bytheir relative volatilities. Other terms used in the industry includefractionation, rectification, and partial condensation.

The second separation unit 20 has an inlet, a first outlet, and a secondoutlet. The inlet is in fluid communication with the product gas header210 of the adsorption separation unit 10. The inlet of the secondseparation unit 20 is operatively disposed to receive at least a portionof the light noble gas-enriched gas 13 from the product gas header 210.A light noble gas-depleted gas 23 is discharged from the first outlet ofthe second separation unit 20. A light noble gas-rich gas 25 isdischarged from the second outlet of the second separation unit 20.

The present apparatus may comprise a gas mixer 60. The gas mixer 60 hasa first inlet for receiving a stream of the feed gas 11, a second inletin fluid communication with a source of a second gas 17, and an outlet.The gas mixer 60 may be a mixing tee, mixing vessel, static mixer, orany other suitable mixing device capable of combining multiple streamsto form a blended stream comprising the multiple streams. The secondinlet of the gas mixer 60 receives a second gas 17 from a source of thesecond gas where the second gas has a higher light noble gas contentthan the feed gas 11. The source of the second gas may be the firstoutlet of the second separation unit 20. The second gas 17 may comprisethe light noble gas-depleted gas 23 from the second separation unit 20.The apparatus may comprise a compressor 45 to compress the light noblegas-depleted gas 23 before the light noble gas-depleted gas 23 is fed tothe gas mixer 60. A separation unit feed gas 12 is formed from the feedgas 11 and the light noble gas-depleted gas 23 and is discharged fromthe outlet of the gas mixer 60 and passed to the feed gas header 200 ofthe adsorption separation unit 10.

The feed gas header 200 of the adsorption separation unit 10 is indownstream fluid communication with the outlet of the gas mixer 60. Thefeed gas header 200 of the adsorption separation unit 10 is operativelydisposed to receive the separation unit feed gas 12 from the gas mixer60.

The present apparatus may comprise a sensor 50 operatively disposed todetect a measure of the helium content and transmit signals responsiveto the helium content. The sensor 50 may be located in a process gastransfer line 11 supplying the first inlet of the gas mixer 60. At thislocation, the sensor 50 would be operatively disposed to detect ameasure of the light noble gas content in the feed gas 11. As shown inFIG. 1, the sensor may be located in a process gas transfer line 12connecting the outlet of the gas mixer 60 to the feed gas header 200 ofthe adsorption separation unit 10. At this location, the sensor 50 isoperatively disposed to detect a measure of the light noble gas contentin the separation unit feed gas 12. Alternatively, the sensor 50 may belocated in the feed gas header 200. In this location, the sensor 50would be operatively disposed to detect a measure of the light noble gascontent in the adsorption vessel feed gas 15.

More than one sensor 50 may be used. In case more than one sensor 50 isused, one or more sensors may be present in the process gas transferline 11 supplying the first inlet of the gas mixer 60 and/or the processgas transfer line 12 connecting the outlet of the gas mixer 60 to thefeed gas header 200 of the adsorption separation unit 10 and/or in thefeed gas header 200.

The measure of the light noble gas content may be a light noble gasconcentration. The measured light noble gas concentration may bedetermined by a concentration sensor 50 that detects the light noble gasconcentration. The measured light noble gas concentration may be a molefraction, mass fraction, mole %, mass %, volume %, or any other suitableconcentration unit. The light noble gas concentration may be in anylight noble gas concentration units.

The present apparatus may comprise a controller 80 in signalcommunication with the sensor 50. The signal communication may bewireless or hard-wired. The controller is operable to control operatingconditions of the second separation unit 20 responsive to signals fromthe sensor 50. The second separation unit 20 is controlled to regulatethe flow rate of the light noble gas in the second gas stream 17responsive to the measure of the light noble gas content.

The second inlet of the gas mixer 60 may be in fluid communication withthe second outlet of the second separation unit 20. The gas mixer 60 mayreceive a portion 28 of the light noble gas-rich gas 25 from the secondseparation unit 20.

The second separation unit 20 may comprise a flow regulator 29operatively disposed between the second inlet of the gas mixer 60 andthe second outlet of the second separation unit 20. The controller 80may be operable to control the second separation unit 20 by adjustingthe flow regulator 29. The flow regulator regulates the flow rate of theportion 28 of the light noble gas-rich gas 25 from the second separationunit 20 to the gas mixer 60 to change the content of light noble gasfeed to the adsorption separation unit 10. The controller 80 adjusts theflow regulator 29 responsive to measure of the light noble gas contentas determined by sensor 50.

The source of the second gas 17 may comprise a process gas transfer line36 which operatively connects the product gas header 210 to the inlet tothe second separation unit 20. The second inlet of the gas mixer 60 maybe in fluid communication with the process gas transfer line 36.

The apparatus may comprise a flow regulator 33 operatively disposedbetween the second inlet of the gas mixer 60 and the process gastransfer line 36. The flow regulator 33 may be in signal communicationwith the controller 80. The signal communication may be wireless orhard-wired. The controller 80 may be operable to control the flow rateof the light noble gas from the process gas transfer line 36 byadjusting the flow regulator 33 operatively disposed between the secondinlet of the gas mixer 60 and the process gas transfer line 36. Flowregulator 33 may regulate the flow rate of a portion of a light noblegas-enriched stream from the adsorption separation unit 10 to the secondinlet of the gas mixer 60 to change the content of light noble gas fedto the adsorption separation unit 10. Controller 80 may adjust flowregulator 33 responsive to the measure of the light noble gas content asdetermined by sensor 50.

In another embodiment, the gas mixer 60 may have a third inlet. Thethird inlet may be in fluid communication with the second outlet of thesecond separation unit 20. The third inlet of the gas mixer 60 mayreceive a portion 28 of the light noble gas-rich gas 25 from the secondseparation unit 20.

The second separation unit 20 may comprise a flow regulator 31 in signalcommunication with controller 80 and operatively disposed between thethird inlet of the gas mixer 60 and the second outlet of the secondseparation unit 20. The controller 80 is operable to control the secondseparation unit 20 by adjusting the flow regulator 31. The flowregulator 31 regulates the flow rate of the portion 28 of the lightnoble gas-rich gas 25 from the second separation unit 20 to the thirdinlet of the gas mixer 60 to change the content of light noble gas feedto the adsorption separation unit 10. The controller 80 adjusts the flowregulator 31 responsive to the measure of the light noble gas content asdetermined by sensor 50.

Alternatively, or in addition, the third inlet of the gas mixer 60 maybe in fluid communication with a process gas transfer line 36 whichoperatively connects the product gas header 210 of the adsorptionseparation unit 10 to the inlet to the second separation unit 20.

The apparatus may comprise a flow regulator 37 operatively disposedbetween the third inlet of the gas mixer 60 and the process gas transferline 36. The flow regulator 37 may be in signal communication with thecontroller 80. The signal communication may be wireless or hard-wired.The controller 80 may be operable to control the flow rate of the lightnoble gas from the process gas transfer line 36 by adjusting the flowregulator 37 operatively disposed between the third inlet of the gasmixer 60 and the process gas transfer line 36. Flow regulator 37 mayregulate the flow rate of a portion of a light noble gas-enriched streamfrom the adsorption separation unit 10 to the third inlet of the gasmixer 60 to change the content of light noble gas fed to the adsorptionseparation unit 10. Controller 80 may adjust flow regulator 37responsive to the measure of the light noble gas content as determinedby sensor 50.

The second separation unit 20 may be a membrane-type separation unit.The membrane separation unit can be any membrane device with someselectivity for separating the light noble gas from the other componentsin the feed when a pressure differential is maintained across themembrane. When the light noble gas is helium, the helium permeabilitythrough the membrane is typically greater than that of the othercomponents present in the feed to the membrane. Consequently, theconcentration of helium in the non-permeate stream from the membraneseparation unit is less than its concentration in the feed streamentering the membrane separation unit. Generally, the pressure of thehelium-depleted non-permeate stream is 10-200 kPa absolute lower thanthe feed stream to the membrane separation unit. The light noblegas-rich permeate stream may have a pressure that is 100 kPa to 500 kPaor 100 kPa to 350 kPa lower than the feed stream to the membraneseparation unit. A higher permeability of helium and/or its selectivitythrough the membrane is desirable and results in a beneficial effect onthe performance of the overall system.

When the second separation unit 20 is a membrane-type separation unit,the membrane unit may consist of a single membrane device or,alternatively, several membrane devices configured and operated so as toachieve the separation in the most efficient manner, e.g., a cascade ofmembranes with internal recycle streams between various stages of themembrane unit. Typically, the membrane devices are manufactured inmodules, each having certain semi-permeable membrane areas forpermeation.

Membrane separation units with some selectivity for separating lightnoble gases such as helium and neon are available commercially, forexample, from Air Products, L′Air Liquide, Ube, Cameron, and UOP.

As shown in FIG. 1, the apparatus may comprise a buffer tank 30 and thelight noble gas-enriched stream 13 may be passed from the adsorptionseparation unit 10 to buffer tank 30 before being passed to the secondmembrane separation unit 20. The buffer tank buffers the fluctuation inpressure and light noble gas concentration of the light noblegas-enriched stream 13 from the adsorption separation unit 10. Uniformlight noble gas concentration and pressure improve controllability ofthe second separation unit 20, particularly when the second separationunit is a membrane-type unit.

The membrane-type separation unit may comprise one or more adjustableorifices 26 operative to control a pressure in the membrane-typeseparation unit. The one or more adjustable orifices are operative tocontrol the pressure difference between the second (membrane) separationunit feed gas stream 21 and the light noble gas-rich (permeate) stream25.

The one or more adjustable orifices may be valves or functionallyequivalent means for controlling flow and/or pressure. FIG. 1 shows avalve 26 in fluid communication with the second outlet of the second(membrane) separation unit 20 and a valve 27 in fluid communication withthe first outlet of the second (membrane) separation unit 20. The valves26 and 27 can be adjusted to control the pressure difference between thesecond separation unit feed gas stream 21 and the light noble gas-rich(permeate) stream 25.

The pressure difference between the second (membrane) separation unitfeed gas stream 21 and the light noble gas-rich (permeate) stream 25 maybe increased or decreased by changing the percent open of the adjustableorifice (i.e. valve 27) in fluid communication with the first outlet fordischarging the light noble gas-depleted (non-permeate) stream 23, andthe percent open of the adjustable orifice (i.e. valve 26) in fluidcommunication with the second outlet for discharging the light noblegas-rich (permeate) stream 25.

The one or more adjustable orifices may be a valve or similar devicecapable of controlling pressure in the membrane-type separation unit.The one or more adjustable orifices may be in signal communication withthe controller 80. The controller may be operable to control the secondseparation unit 20 by adjusting the one or more adjustable orifices 26.Increasing the back-pressure on the permeate side of the membrane-typeseparation unit increases the flow rate of light noble gas in the lightnoble gas-depleted (non-permeate) stream. Decreasing the back-pressureon the permeate side of the membrane-type separation unit decreases theflow rate of light noble gas in the light noble gas-depleted(non-permeate) stream.

The membrane-type separation unit may comprise a plurality of membranemodules and one or more control valves that control the fraction ofmembrane modules on-stream. The one or more control valves may be insignal communication with the controller 80. The controller may beoperable to control the second separation unit 20 by adjusting thefraction of membrane modules on-stream. Increasing the fraction ofmembrane modules on-stream decreases the flow rate of light noble gas inthe light noble gas-depleted (non-permeate) stream. Decreasing thefraction of membrane modules on-stream increases the flow rate of lightnoble gas in the light noble gas-depleted (non-permeate) stream.

The membrane-type separation unit may comprise a heat exchanger 40. Theheat exchanger 40 may be operative to control a temperature in thesecond separation unit 20. The heat exchanger is operatively disposed toselectively heat or cool at least a portion of the second separationunit feed gas stream 21 by indirect heat transfer with a heat transfermedium. The heat transfer medium may be a heat transfer fluid.

The heat exchanger 40 may be in signal communication with the controller80. The signal communication may be wireless or hard-wired. Thecontroller 80 may be operable to control the membrane-type separationunit by adjusting the heat duty of the heat exchanger 40. Increasing thetemperature of the stream entering the membrane-type separation unitdecreases the flow rate of light noble gas in the light noblegas-depleted (non-permeate) stream. Decreasing the temperature of thestream entering the membrane-type separation unit increases the flowrate of light noble gas in the light noble gas-depleted (non-permeate)stream.

As shown in FIG. 1, the second separation unit 20 may comprise acompressor 35 to compress the second separation unit feed gas stream 21.

The second separation unit 20 may be an adsorption-type separation unit.

The adsorption-type separation unit may comprise a plurality of vesselswhere each vessel contains a bed of adsorbent. The adsorption-typeseparation unit may comprise one or more control valves that control thefraction of the plurality of vessels that are on-stream. An adsorptionbed is “on-stream” if it is undergoing an adsorption cycle to form thelight noble gas-depleted stream and the light noble gas-rich stream. Anadsorption bed is “off-line” if it is idle while other adsorption bedsin the system are undergoing an adsorption cycle. The one or morecontrol valves may be in signal communication with the controller 80.The signal communication may be wireless or hard-wired.

The source of the second gas may be the first outlet of the secondseparation unit. The controller 80 may be operable to control the flowrate of light noble gas from the first outlet of the second separationunit to the second inlet of the gas mixer 60 by adjusting the fractionof the plurality of vessels on-stream. Increasing the fraction of theplurality of vessels on-stream decreases the flow rate of light noblegas in the light noble gas-depleted stream. Decreasing the fraction ofthe plurality of vessels on-stream increases the flow rate of lightnoble gas in the light noble gas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit where the second separation unit 20 comprises a feed gasheader and one or more adjustable orifices 32 operative to control apressure in the feed gas header. The controller 80 may be operable tocontrol the flow rate of light noble gas from the source of the secondgas 17 to the second inlet of the gas mixer 60 by adjusting the one ormore adjustable orifices 32 operative to control the pressure in thefeed gas header of the second separation unit 20. The one or moreadjustable orifices 32 may be valves. Increasing the pressure in thefeed gas header decreases the flow rate of light noble gas in the lightnoble gas-depleted stream. Decreasing the pressure in the feed gasheader increases the flow rate of light noble gas in the light noblegas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit where the second separation unit 20 comprises a tail gasheader and one or more adjustable orifices 27 operative to control apressure in the tail gas header. The controller 80 may be operable tocontrol the flow rate of light noble gas from the source of the secondgas 17 to the second inlet of the gas mixer 60 by adjusting the one ormore adjustable orifices 27 to control the pressure in the tail gasheader. The one or more adjustable orifices 27 may be valves. Increasingthe pressure in the tail gas header increases the flow rate of lightnoble gas in the light noble gas-depleted stream. Decreasing thepressure in the tail gas header decreases the flow rate of light noblegas in the light noble gas-depleted stream.

The source of the second gas may comprise the second outlet of thesecond separation unit where the second separation unit 20 comprises aproduct gas header and one or more adjustable orifices 29 operative tocontrol a pressure in the product gas header. The controller 80 may beoperable to control the flow rate of light noble gas from the source ofthe second gas 17 to the second inlet of the gas mixer 60 by adjustingthe one or more adjustable orifices 29 to control the pressure in theproduct gas header. Increasing the pressure in the product gas headerdecreases the flow rate of light noble gas in the light noblegas-depleted stream. Decreasing the pressure in the product gas headerincreases the flow rate of light noble gas in the light noblegas-depleted stream.

The second separation unit 20 may be a distillation-type separationunit.

The source of the second gas may be the first outlet of the secondseparation unit 20 where the second separation unit comprises one ormore adjustable orifices 26, 27, 32 in signal communication with thecontroller 80 operative to control a pressure in the second separationunit 20. The controller 80 may be operable to control the flow rate oflight noble gas from the source of the second gas 17 to the second inletof the gas mixer 60 by adjusting the one or more adjustable orifices 26,27, 32 to control the pressure in the second separation unit 20.Increasing the pressure in the distillation-type separation unit (i.e. adistillation column) increases the flow rate of light noble gas in thelight noble gas-depleted stream. Decreasing the pressure in thedistillation-type separation unit (i.e. a distillation column) decreasesthe flow rate of light noble gas in the light noble gas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit (20) where the apparatus further comprises a heatexchanger 40 in signal communication with the controller 80. The heatexchanger 40 may be operative to control a temperature in the secondseparation unit. The controller 80 may be operable to control the flowrate of light noble gas from the source of the second gas 17 to thesecond inlet of the gas mixer (60) by adjusting the heat duty of theheat exchanger 40. Increasing the heat (temperature) from the heatexchanger increases the temperature in the distillation column whichincreases the flow rate of light noble gas in the light noblegas-depleted stream. Decreasing the heat (temperature) from the heatexchanger decreases the temperature in the distillation column whichdecreases the flow rate of light noble gas in the light noblegas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit 20 where the second separation unit 20 comprises one ormore orifices operative to control a reflux ratio in the secondseparation unit 20. The controller 80 may be operable to control theflow rate of light noble gas from the source of the second gas 17 to thesecond inlet of the gas mixer 60 by adjusting the reflux ratio in thesecond separation unit 20. The “reflux ratio” is defined as the molarflow rate of reflux, which is liquid flow to the top stage of thedistillation column, divided by the molar flow rate of the vapor oroverhead product (i.e. distillate withdrawn from the second outlet).Increasing the reflux ratio increases the flow rate of light noble gasin the light noble gas-depleted stream. Decreasing the reflux ratiodecreases the flow rate of light noble gas in the light noblegas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit 20 where the second separation unit 20 comprises one ormore orifices operative to control a distillate to feed ratio in thesecond separation unit 20. The controller 80 may be operable to controlthe flow rate of light noble gas from the source of the second gas 17 tothe second inlet of the gas mixer 60 by adjusting the distillate to feedratio in the second separation unit 20. The “distillate to feed ratio”is defined as the molar flow rate of vapor or overhead product from thedistillation column (i.e. distillate withdrawn from the second outlet),divided by the molar flow rate of feed fed to the distillation column.Increasing the distillate to feed ratio decreases the flow rate of lightnoble gas in the light noble gas-depleted stream. Decreasing thedistillate to feed ratio increases the flow rate of light noble gas inthe light noble gas-depleted stream.

The source of the second gas may be the first outlet of the secondseparation unit (20) where the second separation unit 20 comprises oneor more orifices operative to control a boilup ratio in the secondseparation unit 20. The controller 80 may be operable to control theflow rate of light noble gas from the source of the second gas 17 to thesecond inlet of the gas mixer 60 by adjusting the boilup to feed ratioin the second separation unit 20. The “boilup ratio” is defined as themolar flow rate of boilup, which is the vapor flow rate to the bottomstage of the distillation column divided by the molar flow rate of theliquid or bottom product (i.e. bottoms stream withdrawn from the firstoutlet). Increasing the boilup ratio decreases the flow rate of lightnoble gas in the light noble gas-depleted stream. Decreasing the boilupratio increases the flow rate of light noble gas in the light noblegas-depleted stream

The present process comprises combining the feed gas stream 11 with asecond gas stream 17 to form a first separation unit feed gas stream 12.The second gas stream 17 has higher light noble gas content than thefeed gas stream 11. The feed gas stream 11 may be combined with thesecond gas stream 17 in gas mixer 60. The second gas stream 17 has aflow rate that is regulated by a regulator, for example, a flow controlvalve.

The feed gas stream 11 may be characterized by having a lowconcentration of light noble gas. The feed gas stream may have a molarconcentration of light noble gas ranging from 0.1 mole % light noble gasto 2.0 mole % light noble gas, or ranging from 0.1 mole % light noblegas to 1.0 mole % light noble gas.

The present process comprises separating the first separation unit feedgas stream 12 in an adsorption separation unit 10 to produce a lightnoble gas-enriched stream 13 and the light noble gas-lean stream 14. Theadsorption separation unit 10 is an adsorption-type separation unit.

Adsorption cycles have a cycle time. The cycle time is a well-understoodand conventional term in the art. The adsorption separation unitundergoes a repeated series of cycle steps of a defined adsorptioncycle. The cycle time is the time period required to complete oneadsorption cycle from start to finish.

The cycle time of the adsorption cycle may be set to provide the lightnoble gas-enriched stream 13 having a bulk average (“mixing cup”average) light noble gas concentration ranging from 40 mole % to 90 mole% for the set cycle time. Contrary to conventional operation, the cycletime for the present process is extended to provide lower bulk averagelight noble gas concentrations for the light noble gas-enriched stream13 leaving the adsorption separation unit 10. Operating the adsorptionseparation unit 10 with a longer cycle time increases the recovery ofthe light noble gas. Due to the low concentrations of light noble gas inthe feed gas stream, high recovery is necessary to be commerciallyviable.

The present process comprises separating a second separation unit feedgas stream 21 in a second separation unit 20 to produce the light noblegas-rich stream 25 and a light noble gas-depleted stream 23. The secondseparation unit feed gas stream 21 comprises the light noblegas-enriched stream 13 from the adsorption separation unit 10. Thesecond separation unit 20 may be a membrane-type separation unit, anadsorption-type separation unit, or a distillation-type separation unit.

In the present process, the flow rate of the light noble gas in thesecond gas stream 17 is controlled responsive to a measure of the lightnoble gas content in at least one of the feed gas stream 11, the firstseparation unit feed gas stream 12, or an adsorption vessel feed gasstream 15. The adsorption vessel feed gas stream 15 comprises at least aportion of the first separation unit feed gas stream 12.

The light noble gas content may be expressed as a concentration or arelative amount in the stream.

The measure of the light noble gas content may be determined from ameasurement of the light noble gas content. The measure of the lightnoble gas content may be expressed as a volume, molar, or massconcentration of the light noble gas. The measure of the light noble gascontent may be expressed as a volume, molar, or mass fraction orpercentage of the light noble gas in the mixture.

The measure of light noble gas content in the first separation unit feedgas stream 15 may be the concentration of light noble gas in the feedgas stream 11.

The flow rate of the light noble gas in a stream may be expressed as avolumetric, molar, or mass flow rate.

The feed gas stream 11 may have a total gas molar flow rate, F₁, with anindividual gas component molar flow rate of light noble gas,F_(1, Noble). The second gas stream 17 may have a total gas molar flowrate, F₂, with an individual gas component molar flow rate of lightnoble gas, F_(2, Noble). The process is characterized by recycling agreater molar flow rate of helium to the feed to the adsorptionseparation unit compared to processes described in the prior art. In thepresent process

${\frac{F_{2,{Noble}}}{F_{1,{Noble}}} \geq 1},$where F_(1, Noble) is the molar flow rate of light noble gas in the feedgas stream 11, and F_(2, Noble) is the molar flow rate of light noblegas in the light noble gas-depleted stream 23. Recycling a higher molarflow rate of light noble gas from the second separation unit 20 to thefeed to the adsorption separation unit 10 maintains stable feedconcentration to the adsorption separation unit 10. This enables theadsorption separation unit 10 to provide constant light noble gas-richgas to the second separation unit 20, both in flow and concentration,which is necessary for final light noble gas product purity from thesecond separation unit 20. FIG. 6 shows the effect of changingadsorption feed gas concentration on the adsorption unit productconcentration.

For instance, if the feed concentration of helium in stream 11 were todrop from 2.0% to 0.5% over time, which is common in natural gases, thehelium product purity of the adsorption unit 10 would decrease from 88%to 67%. This decrease in product purity is caused by the constraint ofmaintaining the adsorption unit recovery to maximize helium production.However, this decrease in helium product purity may lead to a decreasein the noble gas rich purity from the second separation unit because itmay not be possible to operate effectively at the lower secondseparation unit helium concentration/purity.

In addition, recycling a higher molar flow rate of light noble gas fromthe second separation unit 20 to the feed of the adsorption-typeseparation unit 10 increases the recovery of that unit. This effectivelyincreases the overall recovery of the system as the adsorption-type unit10 is the only significant source of helium losses.

The process may be characterized by

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}} \geq 1.$The ratio of light noble gas molar flow rates,

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}}$may be less than or equal to 16. At

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}}$greater than 16, significant volumes of light noble gas are recycled tothe adsorption separation unit 10, and at this point the benefit ofrecycling light noble gas deteriorates and there is no further practicalenhancement of the adsorption-type separation unit 10.

Allie (U.S. Pat. No. 8,268,047) provides an example for heliumpurification with two VPSA in series with a first outlet stream returnedto the first separation unit. From the example the ratio of noble gas(helium) flow from the second separation unit to the first divided bythe noble gas flow in the feed

$( \frac{F_{2,{Noble}}}{F_{1,{Noble}}} )$is calculated as 0.528. In U.S. Pat. No. 5,080,694 by Knoblauch et al.,a similar arrangement to Allie, of unit operations with recycle, isutilized again for helium purification. Per the values in Table 5 fromexample provided,

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}}$of Knoblauch is calculated as 0.222. D'Amico et al. (U.S. Pat. No.5,542,966) again utilizes the same arrangement as Allie and Knoblauch etal. for helium purification. The values provided in the example ofD'Amico yield a

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}}$ratio of 0.408. Finally, Choe et al. (U.S. Pat. No. 4,717,407) employ aprocess with a cryogenic-type unit as the first separation unit followedby a membrane as the second separation unit, with a first outlet streamreturned to the first separation unit. In the second example usingvalues in Table 4 for helium purification,

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}}$of Choe is calculated as 0.077.

The flow rate of light noble gas in the second gas stream 17 may beincreased if the light noble gas content is less than a desired lowerlimit.

The flow rate of light noble gas in the second gas stream 17 may bedecreased if the light noble gas content is greater than a desired upperlimit.

The desired lower limit for the light noble gas content of the firstseparation unit feed gas stream 12 may correspond to a light noble gasmole fraction selected in a range from 0.1 mole % to 0.5 mole %. Thedesired upper limit for the light noble gas content of the feed gasstream 12 may correspond to a light noble gas mole fraction selected ina range from 1.0 mole % to 2 mole %.

The second gas stream 17 may comprise the light noble gas-depletedstream 23 from the second separation unit 20. The light noblegas-depleted stream 23 may be compressed in compressor 45. The flow rateof light noble gas in the second stream 17 may be increased or decreasedby controlling operating conditions of the second separation unit 20responsive to the measure of the light noble gas content.

The molar flow rate of light noble gas, F_(2, Noble), in the light noblegas-depleted stream 23 may be increased or decreased in order tomaintain the mole fraction of light noble gas in the first separationunit feed gas stream 12 relatively constant in order to maintainoperation within a desired light noble gas recovery range.

The second gas stream 17 may further comprise a portion 28 of the lightnoble gas-rich stream 25. The flow rate of the light noble gas in thesecond gas stream 17 may be increased by increasing the flow rate of theportion 28 of the light noble gas-rich stream 25 and decreased bydecreasing the flow rate of the portion 28 of the light noble gas-richstream 25.

The portion 28 may be a divided portion or a separated portion of thelight noble gas-rich stream 25.

In the process, the second separation unit may be a membrane separationunit. The membrane separation unit separates a feed stream into anon-permeate stream and a permeate stream. The non-permeate stream isthe light noble gas-depleted stream 23. The permeate stream is the lightnoble gas-rich stream 25.

Controlling the operating conditions of the membrane separation unit maycomprise increasing the flow rate of light noble gas in the light noblegas-depleted stream 23 by decreasing the pressure difference between themembrane separation unit feed gas stream and the light noble gas-richstream. Controlling the operating conditions of the membrane separationunit may comprise decreasing the flow rate of light noble gas in thelight noble gas-depleted stream by increasing the pressure differencebetween the membrane separation unit feed gas stream and the light noblegas-rich stream.

The membrane separation unit may comprise a plurality of membranemodules. Controlling the operating conditions of the membrane separationunit may comprise increasing the flow rate of light noble gas in thelight noble gas-depleted stream 23 by decreasing the number of membranemodules on-stream. Controlling the operating conditions of the membraneseparation unit may comprise decreasing the flow rate of light noble gasin the light noble gas-depleted stream 23 by increasing the number ofmembrane modules on-stream.

As discussed above, the apparatus may comprise a heat exchanger 40. Theoperating conditions of the membrane separation unit 20 may becontrolled by increasing or decreasing the temperature of the membraneseparation unit feed gas stream 21 in a heat exchanger.

Controlling the operating conditions of the membrane separation unit maycomprise increasing the temperature of the membrane separation unit feedgas stream 21 to decrease the flow rate of light noble gas,F_(2, Noble), in the light noble gas-depleted stream 23. Controlling theoperating conditions of the membrane separation unit may comprisedecreasing the temperature of the membrane separation unit feed gasstream 21 to increase the flow rate of light noble gas, F_(2, Noble), inthe light noble gas-depleted stream 23.

In the process, the second separation unit 20 may be an adsorption-typeseparation unit. The adsorption-type separation unit separates a feedstream into a tail gas stream and a product stream. The tail gas streamis the light noble gas-depleted stream 23. The product stream is thelight noble gas-rich stream 25.

The adsorption-type separation unit may operate with an adsorption cyclehaving a cycle time. Controlling the operating conditions of the secondseparation unit 20 may comprise increasing the cycle time of the secondseparation unit 20 to decrease the flow rate of light noble gas in thelight noble gas-depleted stream 23, and/or decreasing the cycle time ofthe second separation unit 20 to increase the flow rate of light noblegas in the light noble gas-depleted stream 23. When the cycle time isincreased, the capacity during production step for the more adsorbablespecies increases, due to the improved efficiency of removing the moreadsorbable species over longer tail gas generation step times. Byimproving the capacity for the more adsorbable species, less of thedesirable, less adsorbable species (light noble gas) is captured andlost during the steps which produce tail gas.

The adsorption-type separation unit may have a feed gas header, andcontrolling the operating conditions of the second separation unit 20may comprise increasing the pressure of the second separation unit feedgas stream 21 in the feed gas header of the second separation unit 20 todecrease the flow rate of the light noble gas in the light noblegas-depleted stream 23, and/or decreasing the pressure of the secondseparation unit feed gas stream 21 in the feed gas header of the secondseparation unit 20 to increase the flow rate of the light noble gas inthe light noble gas-depleted stream 23. When the feed pressure isincreased, the capacity during the production step for the moreadsorbable species increases, due to the improved efficiency of theadsorbent at higher pressures to adsorb more adsorbable species. Byimproving the capacity for the more adsorbable species, less of thedesirable, less adsorbable species (light noble gas) is captured andlost during the steps which produce tail gas.

The adsorption-type separation unit may have a tail gas header, andcontrolling the operating conditions of the second separation unit 20may comprise increasing the pressure of the light noble gas-depletedstream 23 in the tail gas header of the second separation unit 20 toincrease the flow rate of the light noble gas in the light noblegas-depleted stream 23, and/or decreasing the pressure of the lightnoble gas-depleted stream 23 in the tail gas header of the secondseparation unit 20 to decrease the flow rate of the light noble gas inthe light noble gas-depleted stream 23. When the pressure of the lightnoble gas-depleted stream 23 is increased, the capacity during theproduction step for the ore adsorbable species decreases, due to thelower quantity of the more adsorbable species being removed during thetail gas generating steps. By decreasing the capacity for the moreadsorbable species, more of the desirable, less adsorbable species(light noble gas) is captured and lost during the steps which producetail gas.

The adsorption-type separation unit may operate with an adsorption cyclecomprising a blowdown step having a target pressure for the end of theblowdown step, where a blowdown gas stream is formed during the blowdownstep. The blowdown gas may be compressed to form a rinse gas streamand/or the blowdown gas may be passed to the tail gas header.Controlling the operating conditions of the second separation unit 20may comprise increasing the target pressure for the end of the blowdownstep to increase the flow rate of the light noble gas in the light noblegas-depleted stream 23, and/or decreasing the target pressure for theend of the blowdown step to decrease the flow rate of the light noblegas in the light noble gas-depleted stream 23. For the case where theblowdown gas is compressed to form a rinse gas stream, increasing thetarget pressure for the end of the blowdown step results in less of thelight noble gas being removed, captured, and returned to the adsorbentbed on the production step via the rinse step. Therefore, more of thelight noble gas remains in the bed for the following step(s) thatgenerate tail gas (evacuation and/or purge), so this increases theamount of light noble gas in the noble gas depleted stream.

The adsorption-type separation unit may comprise a plurality ofadsorption beds and operate with a plurality of adsorption cycles eachcomprising a feed step. Controlling the operating conditions of thesecond separation unit 20 may comprise changing to an adsorption cyclehaving fewer adsorption beds simultaneously on the feed step to increasethe flow rate of light noble gas in the light noble gas-depleted stream23, and/or changing to an adsorption cycle having a greater number ofadsorption beds simultaneously on the feed step to increase the flowrate of light noble gas in the light noble gas-depleted stream 23. Whenthe number of adsorption beds simultaneously on the feed step isdecreased, the capacity during the production step for the moreadsorbable species decreases, due to the decreased volume of adsorbentavailable on the feed/production step. By decreasing the capacity formore adsorbable species, more of the desirable, less adsorbable species(light noble gas) is captured and lost during the steps which producetail gas.

The adsorption-type separation unit may comprise a plurality ofadsorption beds and operate with a plurality of adsorption cycles somecomprising a pressure equalization step. Controlling the operatingconditions of the second separation unit 20 may comprise changing to anadsorption cycle having fewer or no pressure equalization steps toincrease the flow rate of light noble gas in the light noblegas-depleted stream 23, and/or changing to an adsorption cycle having agreater number of pressure equalization steps to decrease the flow rateof light noble gas in the light noble gas-depleted stream 23. When thenumber of pressure equalizations is decreased, the capacity during theproduction step for the more adsorbable species decreases, due to thedecreased efficiency of removing the more adsorbable species during thepressure equalization steps. By decreasing the capacity for the moreadsorbable species, more of the desirable, less adsorbable species(light noble gas) is captured and lost during the steps that producetail gas.

In the process, the second separation unit 20 may be a distillation-typeseparation unit. The distillation-type separation unit separates a feedstream into a bottoms stream and an overhead or distillate stream. Thebottoms stream is the light noble gas-depleted stream 23. The overheador distillate stream is the light noble gas-rich stream 25.

The distillation-type separation unit may operate at an operatingpressure. Controlling the operating conditions of the second separationunit 20 may comprise decreasing the operating pressure of the secondseparation unit 20 to increase the flow rate of light noble gas in thelight noble gas-depleted stream 23, and/or increasing the operatingpressure of the second separation unit 20 to decrease the flow rate oflight noble gas in the light noble gas-depleted stream 23. When theoperating pressure is increased, the solubility of the light noble gasin the bottoms stream is increased, which increases the flow rate of thelight noble gas in the light noble gas-depleted stream.

The distillation-type separation unit may operate at an operatingtemperature. Controlling the operating conditions of the secondseparation unit 20 may comprise decreasing the operating temperature ofthe second separation unit 20 to increase the flow rate of light noblegas in the light noble gas-depleted stream 23, and/or increasing theoperating temperature of the second separation unit 20 to decrease theflow rate of light noble gas in the light noble gas-depleted stream 23.When the temperature is increased, the solubility of the light noble gasin the bottoms stream is increased, which increases the flow rate of thelight noble gas in the light noble gas-depleted stream.

The distillation-type separation unit may operate with a reflux ratio.Controlling the operating conditions of the second separation unit 20may comprise increasing the reflux ratio of the second separation unit20 to increase the flow rate of light noble gas in the light noblegas-depleted stream 23, and/or decreasing the reflux ratio of the secondseparation unit 20 to decrease the flow rate of light noble gas in thelight noble gas-depleted stream 23. When the reflux ratio is increased,the amount of reflux flow is increased in relation to the amount ofoverhead or distillate flow, which ‘washes’ more light noble gas intoliquid solution in the distillation-type separation unit and thereforethe bottoms stream, thereby increasing light noble gas-depleted streamflow.

The distillation-type separation unit may operate with a distillate tofeed ratio. Controlling the operating conditions of the secondseparation unit 20 may comprise increasing the distillate to feed ratioof the second separation unit 20 to decrease the flow rate of lightnoble gas in the light noble gas-depleted stream 23, and/or decreasingthe distillate to feed ratio of the second separation unit 20 toincrease the flow rate of light noble gas in the light noblegas-depleted stream 23. When the distillate to feed ratio is increased,the amount of distillate flow is increased in relation to the amount offeed flow, which decreases the amount/quantity of light noble gas in thedistillation-type separation unit and therefore the bottoms stream,thereby decreasing light noble gas-depleted stream flow.

The distillation-type separation unit may operate with a boilup ratio.Controlling the operating conditions of the second separation unit 20may comprise increasing the boilup ratio of the second separation unit20 to decrease the flow rate of light noble gas in the light noblegas-depleted stream 23, and/or decreasing the boilup ratio of the secondseparation unit 20 to increase the flow rate of light noble gas in thelight noble gas-depleted stream 23. When the boilup ratio is increased,the amount of vapor flow to the bottom stage is increased in relation tothe amount of bottoms flow, which evaporates the light noble gas fromthe bottoms stream, thereby decreasing light noble gas-depleted streamflow.

The term “reflux ratio” is a standard term in the distillation art andis the ratio of reflux flow rate to overhead or distillate flow rate.The term “distillate to feed ratio” is a standard term in thedistillation art and is the ratio of distillate flow rate to feed flowrate. The term “boilup ratio” is a standard term in the distillation artand is the ratio of boilup or vapor to the bottom stage flow rate tobottoms flow rate.

Any required pretreatment of gaseous feed mixtures to the variousseparation units, or post-treatment of any of the product streams can beemployed with this process and apparatus, as required or desired. Forexample, depending upon the choice of the adsorbents used, apretreatment to remove certain components from the feed gas stream 11which might have an adverse effect on the adsorbent or process may berequired. Similarly, it is possible to have components in the finalhelium product, i.e. the helium-rich permeate stream 25, which may beundesirable in the subsequent use of this product stream and must beremoved in a post-treatment operation prior to its use.

EXAMPLES Example 1

A multiple-bed adsorption pilot unit/experimental apparatus was set upto gather light noble gas recovery data for an adsorption separationunit. The unit consisted of 5 adsorption beds, each 2.21 cm (0.87 in.)inside diameter by 3.05 m (10 ft.) in length. The adsorption cycleutilized is shown in FIG. 2, which is a 5-bed vacuum swing adsorption(VSA) with a rinse step (described above). The beds were filled withactivated carbon adsorbent and the cycle time onstream varied between 30seconds, 60 seconds, and 120 seconds. The feed composition to the unitwas varied between 0.35 mole % He to 4 mole % He with nitrogen, methane,and carbon dioxide making up the balance of the feed gas. The feedpressure was varied between 345 kPa (absolute) and 1029 kPa (absolute)and the feed temperature was 21.1° C. (70° F.). The optional buffer tank30 was included in the experimental set up and product purity wasmeasured at the outlet from this tank, which is equivalent to secondseparation unit inlet stream 21 in FIG. 1.

In the first experiment, varying amounts/concentrations of helium werefed to the experimental apparatus and the overall helium recovery wasmeasured. The curve titled “AP Experimental System” in FIG. 4 shows theresults of varying feed helium mole % to the adsorption unit with 70mole % helium product purity and a 1:1 Rinse to Feed ratio for all datapoints on said curve. From FIG. 4, it can be seen that operating on theright side (higher helium feed gas concentrations) of FIG. 4 is morebeneficial to overall system helium recovery. In practice, the feed gashelium concentration in stream 11 of FIG. 1 varies and declines withtime.

Therefore, maintaining the helium concentration to the adsorption unitbrings value and benefit. Higher helium concentration to the adsorptionunit could be achieved by recycling helium from a second downstreamseparation unit. However, recycling helium from a second downstreamseparation unit is counter-intuitive because the (first) adsorptionseparation unit is the only source of helium losses, leading to lowerhelium recovery. But if helium recovery from the (first) adsorptionseparation unit could be improved to the point where benefits ofadditional helium feed concentration outweigh the helium losses, thiswould provide non-obvious benefits.

In another set of experiments, the experimental system was operated withvarying secondary volume, V₂. FIG. 5 shows experimental results of theadsorption-type system in the present invention on helium recovery as afunction of varying secondary volume. Data in FIG. 5 is with 2 mole %helium in the feed and a 1:1 Rinse to Feed ratio. Rinse to Feed ratio isa measure of the molar flow of gas (stream 18 FIG. 1) taken from the bedduring the Rinse step divided by the molar flow of feed gas (stream 12in FIG. 1) gas sent to the bed in the Feed step. This ratio is monitoredand controlled by the pressure in the bed at the end of the Blowdownstep. In the experimental system, this was performed by adding isolationvalves to the feed and product end of the column/bed, effectivelyreducing secondary volume. From the FIG. 5, it is clear that decreasingsecondary volume will increase helium recovery.

From the experimental results, another unexpected benefit to maintainingadsorption unit feed (stream 12 in FIG. 1) is observed, which isillustrated in FIG. 6. As the adsorption unit feed gas (stream 12 inFIG. 1) declines, the helium product purity from the adsorption unit(stream 21 in FIG. 1) also declines. Since the feed concentration isknown to vary and decline with time, this variation with product purityof the second unit inlet stream 21 may lead to difficulty controllingthe second separation unit. Additionally, since there is afinite/practice design basis for the second separation unit, thesevariations could lead to a decline of light noble gas rich stream 25.Decline of the light noble gas rich stream 25 below the required purityfor light noble gas might force the unit to be significantly turned downor potentially shut down.

Example 2

The present invention aims to maximize light noble gas recovery whilehandling varying and low concentrations of light noble gas in the feedby recycling a second stream rich in light noble gas to enrich the lightnoble gas content in the feed to the adsorption separation unit. Fromthe findings in the experimental system, there has been an intentionaleffort to maximize helium recovery over the prior art by decreasing thesecondary volumes of the adsorption unit. The curve titled “HeaderChanges” in FIG. 4 shows the impact from simulation modeling of reducingthe secondary volumes by 50% over a typical commercial system byre-arranging and/or minimizing piping volumes.

The curve titled “Isolation Valve” in FIG. 4 employs an isolation switchvalve in fluid communication with the bed at feed and product end of theadsorbent bed. This significantly reduces the secondary volume over the“Header Change” and “Commercial System”, which in turn improves theadsorption and overall system helium recovery. The isolation valve onthe product header opens during Production (P), Rinse (R), and ProductPressurization (PP) but closes during Blowdown (BD) and Evac (E). Theisolation on the feed header closes during Product Pressurization (PP)and is open during all other adsorption steps to prevent gas having highhelium concentrations from entering the secondary volumes on the feedend of the bed.

By reducing the secondary volume by header changes or isolation valves,the helium recovery at all combined gas stream 12 concentrations issignificantly improved over the prior art and enables the presentinvention to achieve higher recovery across the whole range of heliumconcentration of interest (0.1% to 4%), as shown in FIG. 4.

Example 3

Greater benefits are seen in the case when the helium content of theincoming feed gas stream 11 drops from 4% to 0.5% over time. The priorart in this example is U.S. Pat. No. 8,268,047, a double VPSA for heliumrecovery. This change in the helium content of the feed gas stream 11will cause a decrease in the helium purity of the adsorption unitenriched gas stream 13 as shown in FIG. 6.

Due to the lower incoming purity to the second stage inlet, the priorart, given a fixed system already in operation, would have to ‘shortcycle’ (well known to someone skilled in the art of adsorption) thefinal purification stage to maintain final helium purity. Thisinherently increases the second stream helium flow (F_(2, Noble)) whichleads to an increased helium loss as evidenced on the curve titled“Prior Art” in the FIG. 7.

In the current invention, due to the minimization of secondary volume inthe adsorption separation unit, the adsorption separation unit recoveryis implicitly higher, which enables the ability to actively increase the

$\frac{F_{2,{Noble}}}{F_{1,{Noble}}} \geq 1$as the feed content drops. This yields a significant improvement inhelium recovery compared to the Prior Art as the feed content drops, asshown in FIG. 7.

We claim:
 1. An apparatus for producing a light noble gas product from afeed gas comprising a light noble gas and at least one other gaseouscomponent, the light noble gas selected from the group consisting ofhelium, and neon, the apparatus comprising: an adsorption separationunit, wherein the adsorption separation unit comprises a plurality ofvessels each containing a bed of adsorbent; a feed gas header inselective fluid communication with each of the plurality of vessels; aproduct gas header in selective fluid communication with each of theplurality of vessels; a tail gas header in selective fluid communicationwith each of the plurality of vessels; process gas transfer linesoperatively connecting the plurality of vessels to the feed gas header,the product gas header, and the tail gas header; each vessel of theplurality of vessels having process gas transfer lines associatedtherewith; a plurality of valves in the process gas transfer linesincluding a plurality of valves adjacent and associated with eachrespective vessel; wherein the adsorption separation unit has a centralvolume of process gas transfer lines associated with each of therespective vessels, V_(c); wherein the central volume for eachrespective vessel is the sum of (i) the volume contained in the processgas transfer lines associated with the respective vessel connecting therespective vessel to each valve adjacent the respective vessel, (ii) alldead-end volumes, if any, connected at a junction to the respectivevessel, and (iii) all dead-end volumes, if any, connected at a junctionto any of the process gas transfer lines associated with the respectivevessel that connect the respective vessel to any valve adjacent therespective vessel; wherein the central volume for each respective vesselincludes a secondary volume, V₂, where the secondary volume is the sumof (i) the volume of all dead-end volumes, if any, connected to therespective vessel; (ii) the volume of all dead-end volumes, if any,connected at a junction to any of the process gas transfer linesassociated with the respective vessel that connect the respective vesselto any valve adjacent the respective vessel, and (iii) the volume of anyprocess gas transfer lines, if any, having a first end terminating in avalve adjacent the respective vessel that is configured to permittransfer of process gas to the tail gas header when open and having asecond end terminating at a junction to any other of the associatedprocess gas transfer lines that connect the respective vessel to anyother valve adjacent the respective vessel; and wherein the secondaryvolume V₂ is less than 5% of the central volume V_(c) for each vessel.2. The apparatus according to claim 1 further comprising: a secondseparation unit, the second separation unit having an inlet, a firstoutlet, and a second outlet, the inlet in fluid communication with theproduct gas header of the adsorption separation unit; a gas mixer havinga first inlet for receiving a stream of the feed gas, a second inlet influid communication with a source of a second gas having a higher lightnoble gas concentration than the feed gas, wherein the feed gas headerof the adsorption separation unit is in downstream fluid communicationwith the outlet of the gas mixer; a sensor in at least one of (i) aprocess gas transfer line supplying the first inlet of the gas mixer,(ii) a process gas transfer line connecting the outlet of the gas mixerto the feed gas header of the adsorption separation unit, and (iii) thefeed gas header; and a controller in signal communication with thesensor, the controller operable to control the flow rate of light noblegas from the source of the second gas to the second inlet of the gasmixer responsive to signals from the sensor.
 3. The apparatus accordingto claim 2 wherein the source of the second gas comprises the firstoutlet of the second separation unit.
 4. The apparatus according toclaim 2 wherein the source of the second gas comprises the second outletof the second separation unit.
 5. The apparatus according to claim 2wherein the source of the second gas comprises a process gas transferline which operatively connects the product gas header to the inlet tothe second separation unit.
 6. The apparatus according to claim 2wherein the gas mixer has a third inlet in fluid communication with thesecond outlet of the second separation unit.
 7. The apparatus accordingto claim 2 wherein the gas mixer has a third inlet in fluidcommunication with a process gas transfer line which operativelyconnects the product gas header of the adsorption separation unit to theinlet to the second separation unit.
 8. The apparatus according to claim2 wherein the second separation unit is a membrane-type separation unit,wherein the source of the second gas comprises the first outlet of thesecond separation unit; and wherein at least one of (a) the secondseparation unit comprises one or more adjustable orifices in signalcommunication with the controller, the one or more adjustable orificesoperative to control a pressure in the second separation unit; and thecontroller is operable to control the flow rate of light noble gas fromthe source of the second gas to the second inlet of the gas mixer byadjusting the one or more adjustable orifices; (b) the membrane-typeseparation unit comprises a plurality of membrane modules and one ormore control valves that control the fraction of membrane moduleson-stream, the one or more control valves in signal communication withthe controller; and the controller is operable to control the flow rateof light noble gas from the source of the second gas to the second inletof the gas mixer by adjusting the fraction of membrane moduleson-stream; or (c) the apparatus comprises a heat exchanger operative tocontrol a temperature in the second separation unit, the heat exchangerin signal communication with the controller; and the controller isoperable to control the flow rate of light noble gas from the source ofthe second gas to the second inlet of the gas mixer by adjusting theheat duty of the heat exchanger.
 9. The apparatus according to claim 2wherein the second separation unit is an adsorption-type separationunit, wherein at least one of (a) the source of the second gas comprisesthe first outlet of the second separation unit; the adsorption-typeseparation unit comprises a plurality of vessels each containing a bedof adsorbent, and one or more control valves that control the fractionof the plurality of vessels on-stream, the one or more control valves insignal communication with the controller; and the controller is operableto control the flow rate of light noble gas from the source of thesecond gas to the second inlet of the gas mixer by adjusting thefraction of the plurality of vessels on-stream; (b) the source of thesecond gas comprises the first outlet of the second separation unit; thesecond separation unit comprises a feed gas header, the secondseparation unit comprises one or more adjustable orifices operative tocontrol a pressure in the feed gas header of the second separation unit;and the controller is operable to control the flow rate of light noblegas from the source of the second gas to the second inlet of the gasmixer by adjusting the one or more adjustable orifices operative tocontrol the pressure in the feed gas header of the second separationunit; (c) the source of the second gas comprises the first outlet of thesecond separation unit; the second separation unit comprises a tail gasheader, the second separation unit comprises one or more adjustableorifices operative to control a pressure in the tail gas header of thesecond separation unit; and the controller is operable to control theflow rate of light noble gas from the source of the second gas to thesecond inlet of the gas mixer by adjusting the one or more adjustableorifices operative to control the pressure in the tail gas header of thesecond separation unit; or (d) the source of the second gas comprisesthe second outlet of the second separation unit; the second separationunit comprises a product gas header, the second separation unitcomprises one or more adjustable orifices operative to control apressure in the product gas header of the second separation unit; andthe controller is operable to control the flow rate of light noble gasfrom the source of the second gas to the second inlet of the gas mixerby adjusting the one or more adjustable orifices operative to controlthe pressure in the product gas header of the second separation unit.10. The apparatus according to claim 2 wherein the second separationunit is a distillation-type separation unit, wherein the source of thesecond gas comprises the first outlet of the second separation unit; andwherein at least one of (a) the second separation unit comprises one ormore adjustable orifices in signal communication with the controller,the one or more adjustable orifices operative to control a pressure inthe second separation unit; and the controller is operable to controlthe flow rate of light noble gas from the source of the second gas tothe second inlet of the gas mixer by adjusting the one or moreadjustable orifices operative to control the pressure in the secondseparation unit; (b) the apparatus comprises a heat exchanger operativeto control a temperature in the second separation unit, the heatexchanger in signal communication with the controller; and thecontroller is operable to control the flow rate of light noble gas fromthe source of the second gas to the second inlet of the gas mixer byadjusting the heat duty of the heat exchanger; (c) the second separationunit comprises one or more orifices operative to control a reflux ratioin the second separation unit; and the controller is operable to controlthe flow rate of light noble gas from the source of the second gas tothe second inlet of the gas mixer by adjusting the reflux ratio in thesecond separation unit; (d) the second separation unit comprises one ormore orifices operative to control a distillate to feed ratio in thesecond separation unit; and the controller is operable to control theflow rate of light noble gas from the source of the second gas to thesecond inlet of the gas mixer by adjusting the distillate to feed ratioin the second separation unit; or (e) the second separation unitcomprises one or more orifices operative to control a product to feedratio in the second separation unit; and the controller is operable tocontrol the flow rate of light noble gas from the source of the secondgas to the second inlet of the gas mixer by adjusting the distillate tofeed ratio in the second separation unit.
 11. A process for separating afeed gas stream comprising a light noble gas and at least one othergaseous component into a light noble gas-rich stream and a light noblegas-lean stream, the light noble gas selected from the group consistingof helium, and neon, the process comprising: combining the feed gasstream with a second gas stream to form a combined gas stream, thesecond gas stream having higher light noble gas content than the feedgas stream, the second gas stream having a flow rate that is regulated;separating a first separation unit feed gas stream in an adsorptionseparation unit to produce a light noble gas-enriched stream and thelight noble gas-lean stream, wherein the first separation unit feed gasstream comprises at least a portion of the combined gas stream; andseparating a second separation unit feed gas stream in a secondseparation unit to produce the light noble gas-rich stream and a lightnoble gas-depleted stream, wherein said second separation unit feed gasstream comprises at least a portion of the light noble gas-enrichedstream from the adsorption separation unit; wherein the flow rate of thelight noble gas in the second gas stream is controlled responsive to ameasure of the light noble gas content in at least one of the feed gasstream, the combined gas stream, or the first separation unit feed gasstream.
 12. The process according to claim 11 wherein the adsorptionseparation unit comprises: a plurality of vessels each containing a bedof adsorbent; a feed gas header in selective fluid communication witheach of the plurality of vessels; a product gas header in selectivefluid communication with each of the plurality of vessels; a tail gasheader in selective fluid communication with each of the plurality ofvessels; process gas transfer lines operatively connecting the pluralityof vessels to the feed gas header, the product gas header, and the tailgas header; each vessel of the plurality of vessels having process gastransfer lines associated therewith; a plurality of valves in theprocess gas transfer lines including a plurality of valves adjacent andassociated with each respective vessel; wherein the adsorptionseparation unit has a central volume of process gas transfer linesassociated with each of the respective vessels, V_(c); wherein thecentral volume for each respective vessel is the sum of (i) the volumecontained in the process gas transfer lines associated with therespective vessel connecting the respective vessel to each valveadjacent the respective vessel, (ii) all dead-end volumes, if any,connected at a junction to the respective vessel, and (iii) all dead-endvolumes, if any, connected at a junction to any of the process gastransfer lines associated with the respective vessel that connect therespective vessel to any valve adjacent the respective vessel; whereinthe central volume for each respective vessel includes a secondaryvolume, V₂, where the secondary volume is the sum of (i) the volume ofall dead-end volumes, if any, connected to the respective vessel; (ii)the volume of all dead-end volumes, if any, connected at a junction toany of the process gas transfer lines associated with the respectivevessel that connect the respective vessel to any valve adjacent therespective vessel, and (iii) the volume of any process gas transferlines, if any, having a first end terminating in a valve adjacent therespective vessel that is configured to permit transfer of process gasto the tail gas header when open and having a second end terminating ata junction to any other of the associated process gas transfer lines(102) that connect the respective vessel to any other valve adjacent therespective vessel; and wherein the secondary volume V₂ is less than 5%of the central volume V_(c) for each vessel.
 13. The process accordingto claim 11 where the feed gas stream has a total gas molar flow rate,F₁, with a molar flow rate of light noble gas, F_(1, Noble), and thesecond gas stream has a total gas molar flow rate, F₂, with a molar flowrate of light noble gas, F_(2, Noble), and wherein$\frac{F_{2,{Noble}}}{F_{1,{Noble}}} \geq 1.$
 14. The process accordingto claim 11 wherein the flow rate of light noble gas in the second gasstream is increased if the light noble gas content is less than adesired lower limit; and/or wherein the flow rate of light noble gas inthe second gas stream is decreased if the light noble gas content isgreater than a desired upper limit.
 15. The process according to claim11 wherein the second gas stream comprises the light noble gas-depletedstream, and wherein the flow rate of light noble gas in the secondstream is increased or decreased by controlling operating conditions ofthe second separation unit in response to the light noble gas content.16. The process according to claim 15 wherein the second separation unitis a membrane-type separation unit comprising a plurality of membranemodules and wherein controlling operating conditions of the secondseparation unit comprises at least one of (a) decreasing the pressuredifference between the second separation unit feed gas stream and thelight noble gas-rich stream to increase the flow rate of light noble gasin the light noble gas-depleted stream; and/or increasing the pressuredifference between the second separation unit feed gas stream and thelight noble gas-rich stream to decrease the flow rate of light noble gasin the light noble gas-depleted stream; (b) decreasing the number ofmembrane modules on-stream to increase the flow rate of light noble gasin the light noble gas-depleted stream; and/or increasing the number ofmembrane modules on-stream to decrease the flow rate of light noble gasin the light noble gas-depleted stream; or (c) increasing thetemperature of the second separation unit feed gas stream to decreasethe flow rate of light noble gas in the light noble gas-depleted stream;and/or decreasing the temperature of the second separation unit feed gasstream to increase the flow rate of light noble gas in the light noblegas-depleted stream.
 17. The process according to claim 15 wherein thesecond separation unit is an adsorption-type separation unit wherein atleast one of (a) the adsorption-type separation unit operates with anadsorption cycle having a cycle time and controlling the operatingconditions of the second separation unit comprises increasing the cycletime of the second separation unit to decrease the flow rate of lightnoble gas in the light noble gas-depleted stream; and/or decreasing thecycle time of the second separation unit to increase the flow rate oflight noble gas in the light noble gas-depleted stream; (b) theadsorption-type separation unit has a feed gas header and controllingthe operating conditions of the second separation unit comprisesincreasing the pressure of the second separation unit feed gas stream inthe feed gas header of the second separation unit to decrease the flowrate of the light noble gas in the light noble gas-depleted stream;and/or decreasing the pressure of the second separation unit feed gasstream in the feed gas header of the second separation unit to increasethe flow rate of the light noble gas in the light noble gas-depletedstream; (c) the adsorption-type separation unit has a tail gas headerand controlling the operating conditions of the second separation unitcomprises increasing the pressure of the light noble gas-depleted streamin the tail gas header of the second separation unit to increase theflow rate of the light noble gas in the light noble gas-depleted stream;and/or decreasing the pressure of the light noble gas-depleted stream inthe tail gas header of the second separation unit to decrease the flowrate of the light noble gas in the light noble gas-depleted stream; (d)the adsorption-type separation unit operates with an adsorption cyclecomprising a blowdown step having a target pressure for the end of theblowdown step and controlling the operating conditions of the secondseparation unit comprises increasing the target pressure for the end ofthe blowdown step to increase the flow rate of the light noble gas inthe light noble gas-depleted stream; and/or decreasing the targetpressure for the end of the blowdown step to decrease the flow rate ofthe light noble gas in the light noble gas-depleted stream; (e) theadsorption-type separation unit comprises a plurality of adsorption bedsand operates with a plurality of adsorption cycles each comprising afeed step and controlling the operating conditions of the secondseparation unit comprises changing to an adsorption cycle having alesser number of adsorption beds simultaneously on the feed step toincrease the flow rate of light noble gas in the light noblegas-depleted stream; and/or changing to an adsorption cycle having agreater number of adsorption beds simultaneously on the feed step toincrease the flow rate of light noble gas in the light noblegas-depleted stream; or (f) the adsorption-type separation unitcomprises a plurality of adsorption beds and operates with a pluralityof adsorption cycles some comprising a pressure equalization step andcontrolling the operating conditions of the second separation unitcomprises: changing to an adsorption cycle having a lesser number of orno pressure equalization steps to increase the flow rate of light noblegas in the light noble gas-depleted stream; and/or changing to anadsorption cycle having a greater number of pressure equalization stepsto decrease the flow rate of light noble gas in the light noblegas-depleted stream.
 18. The process according to claim 15 wherein thesecond separation unit is a distillation-type separation unit operatingwith a reflux ratio and at an operating pressure and an operatingtemperature, wherein controlling the operating conditions of the secondseparation unit comprises at least one of (a) decreasing the operatingpressure of the second separation unit to increase the flow rate oflight noble gas in the light noble gas-depleted stream; and/orincreasing the operating pressure of the second separation unit todecrease the flow rate of light noble gas in the light noblegas-depleted stream; (b) increasing the reflux ratio of the secondseparation unit to increase the flow rate of light noble gas in thelight noble gas-depleted stream; and/or decreasing the reflux ratio ofthe second separation unit to decrease the flow rate of light noble gasin the light noble gas-depleted stream; or (c) decreasing the operatingtemperature of the second separation unit to increase the flow rate oflight noble gas in the light noble gas-depleted stream; and/orincreasing the operating temperature of the second separation unit todecrease the flow rate of light noble gas in the light noblegas-depleted stream.
 19. The process according to claim 11 wherein thesecond gas stream comprises a portion of the light noble gas-rich streamhaving a flow rate, and the flow rate of the light noble gas in thesecond gas steam is increased by increasing the flow rate of the portionof the light noble gas-rich stream and decreased by decreasing the flowrate of the portion of the light noble gas-rich stream.
 20. The processaccording to claim 11 wherein the feed gas stream has a molarconcentration of light noble gas ranging from 0.1 mole % light noble gasto 2.0 mole % light noble gas.